drstrace_unit_tests is failing relatively consistently

The drstrace_unit_tests test has been failing for a few weeks now in all
builds on a fairly regular basis:

```
"drstrace_unit_tests" start time: Oct 31 03:31 Eastern Daylight Time
Output:
----------------------------------------------------------
drfront_fetch_module_symbols failed 1 for |D:/derek/drmemory/nightly/run/build_drmemory-rel-32/bin/symfetch.dll| in dir ||

Test time = 37.42 sec
```

Xref #1612: drstrace module fetch fails when symfetch.dll is built with VS2013

When I try to repro, It's non-det: sometimes works, sometimes fails:

```
Testing symbol library fetching features
Using symbol path D:\derek\drmemory\git\build_x86_dbg\logs\drstrace_unit_tests_6096\symbols as the local store
Using symbol path srv*D:\derek\drmemory\git\build_x86_dbg\logs\drstrace_unit_tests_6096\symbols*http://msdl.microsoft.com/download/symbols to fetch symbols
drfront_fetch_module_symbols for |D:/derek/drmemory/git/build_x86_dbg/bin/symfetch.dll| in dir |D:\derek\drmemory\git\build_x86_dbg\logs\drstrace_unit_tests_6096|
failed to fetch pdb for D:/derek/drmemory/git/build_x86_dbg/bin/symfetch.dll, got SymType 0
drfront_fetch_module_symbols failed 1 for |D:/derek/drmemory/git/build_x86_dbg/bin/symfetch.dll| in dir ||
```

SymNone = 0,

Maybe a network issue? Xref MS symbol server failures recently with win10 where a 2nd try works. My theory is that this is a MS symbol server problem and nothing to do with DrM code or this particular hardware. Maybe we should have the test just try again if the fetch fails?