[cygwin] condition_variable::timed_wait tests fail in test_mutex

In test_mutex there are some simple tests that lock a mutex, then call timed_wait on a condition variable and the mutex. Given no other code will notify the condition variable, the tests (test_mutex.cpp:52 and test_mutex.cpp:91) expect to result to be false. On cygwin the result is sometimes true:

https://ci.appveyor.com/project/jeking3/thread/builds/20524620/job/axu9rgvpm2h1djgm?fullLog=true#L1626

I was able to reproduce this locally in the debugger by modifying timed_wait such that I could place a breakpoint on the failure:
```
Thread 17 "test_mutex" hit Breakpoint 1, boost::condition_variable_any::timed_wait > (this=0xffc4cb2c, m=..., abs_time=...)
at ../../../boost/thread/pthread/condition_variable.hpp:249
249 return result || true;
(gdb) l
244 detail::real_platform_timepoint before = detail::real_platform_clock::now();
245 const detail::platform_duration d(ts - before);
246 bool result = do_wait_until(m, detail::internal_platform_clock::now() + d);
247 detail::real_platform_timepoint after = detail::real_platform_clock::now();
248 if (ts > after)
249 return result || true;
250 return false;
251 #else
252 return do_wait_until(m, ts);
253 #endif
(gdb) p before
$1 = {dur = {ts_val = {tv_sec = 1543097563, tv_nsec = 498393000}}}
(gdb) p d
$2 = {ts_val = {tv_sec = 0, tv_nsec = 598000000}}
(gdb) p after
$3 = {dur = {ts_val = {tv_sec = 1543097564, tv_nsec = 96388700}}}
(gdb) p result
$4 = false
(gdb) p after - before
[New Thread 27380.0x74a4]
$5 = {ts_val = {tv_sec = 0, tv_nsec = 597995700}}
(gdb) p ts
$6 = {dur = {ts_val = {tv_sec = 1543097564, tv_nsec = 96393000}}}
```
The `after` time is 4300us before the end of `ts` which is the time point that should have passed. This likely means there is a rounding error somewhere, either in the time handling of boost.thread or down in cygwin.