tests take too long to complete on variant=debug

Typed `b2 -j2 variant=debug threading=multi link=shared address-model=64 toolset=msvc-14.1 warnings=off libs/sort/test//test_parallel_stable_sort` and it took about 5-7 minutes to complete (with `variant=release` it is done in seconds).

It does build with optimizations though:
```
Usage requirements for test_parallel_stable_sort: .
Build properties: ms 64 x86 off pe fcontext object on 32 x86 BOOST_ALL_NO_LIB=1 no xsltproc on on off html true windows object(notfile-target)@9043 . .. off off shared hidden test_parallel_stable_sort.test yes yes speed NT on on off off C:\Users\SERPENT\AppData\Local\Programs\Python\Python37\python 3.7 build:address-model build:architecture build:cxxstd build:cxxstd-dialect build:target-os build:toolset build:toolset-msvc:vendor build:toolset-msvc:version define:toolset link:toolset python.interpreter:address-model python.interpreter:python python.interpreter:target-os threading:runtime-link threading:toolset toolset on on shared native off false project-relative @Jamfile%Jamfile.tag windows on win32 multi 14.1 msvc console debug off hidden off off desktop boost.defaults=Boost
Usage requirements from test_parallel_stable_sort: . address-model architecture asynch-exceptions context-impl cxxstd cxxstd-dialect debug-store debug-symbols deduced-address-model deduced-architecture embed-manifest exception-handling extern-c-nothrow htm inlining instruction-set known-warnings link numa optimization pch pch-on:version preserve-test-targets rtti runtime-debugging runtime-link segmented-stacks target-os testing.execute threading toolset toolset-msvc:vendor toolset-msvc:version valgrind variant warnings warnings-as-errors wavetool windows-api
common.mkdir bin.v2\libs\sort

if not exist "bin.v2\libs\sort\\" mkdir "bin.v2\libs\sort"

common.mkdir bin.v2\libs\sort\test

if not exist "bin.v2\libs\sort\test\\" mkdir "bin.v2\libs\sort\test"

common.mkdir bin.v2\libs\sort\test\test_parallel_stable_sort.test

if not exist "bin.v2\libs\sort\test\test_parallel_stable_sort.test\\" mkdir "bin.v2\libs\sort\test\test_parallel_stable_sort.test"

common.mkdir bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1

if not exist "bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\\" mkdir "bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1"

common.mkdir bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug

if not exist "bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\\" mkdir "bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug"

common.mkdir bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64

if not exist "bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\\" mkdir "bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64"

common.mkdir bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed

if not exist "bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\\" mkdir "bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed"

common.mkdir bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi

if not exist "bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi\\" mkdir "bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi"

file bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi\test_parallel_stable_sort.obj.rsp
"libs\sort\test\test_parallel_stable_sort.cpp" -Fo"bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi\test_parallel_stable_sort.obj" -TP /O2 /Z7 /Ob0 /W0 /GR /MDd /Zc:forScope /Zc:wchar_t /wd4996 /favor:blend /wd4675 /EHs -c
-DBOOST_ALL_NO_LIB=1
"-I."
"-I.."
compile-c-c++ bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi\test_parallel_stable_sort.obj

call "bin.v2\standalone\msvc\msvc-14.1\address-model-64\architecture-x86\msvc-setup.bat" >nul
cl /Zm800 -nologo @"bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi\test_parallel_stable_sort.obj.rsp"

test_parallel_stable_sort.cpp

file bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi\test_parallel_stable_sort.exe.rsp

"bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi\test_parallel_stable_sort.obj"
msvc.link bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi\test_parallel_stable_sort.exe

call "bin.v2\standalone\msvc\msvc-14.1\address-model-64\architecture-x86\msvc-setup.bat" >nul
link /NOLOGO /INCREMENTAL:NO /DEBUG /MACHINE:X64 /MANIFEST /subsystem:console /out:"bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi\test_parallel_stable_sort.exe" @"bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi\test_parallel_stable_sort.exe.rsp"
if %ERRORLEVEL% NEQ 0 EXIT %ERRORLEVEL%

msvc.manifest bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi\test_parallel_stable_sort.exe

if exist "bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi\test_parallel_stable_sort.exe.manifest" (
call "bin.v2\standalone\msvc\msvc-14.1\address-model-64\architecture-x86\msvc-setup.bat" >nul
mt -nologo -manifest "bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi\test_parallel_stable_sort.exe.manifest" "-outputresource:bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi\test_parallel_stable_sort.exe;1"
)

testing.capture-output bin.v2\libs\sort\test\test_parallel_stable_sort.test\msvc-14.1\debug\address-model-64\optimization-speed\threading-multi\test_parallel_stable_sort.run
```