cond_var test failure on AppVeyor

[marcinslusarz]

18/58 Test #20: cond_var_0_none ...................................................***Failed    1.73 sec
-- Executing:  C:/projects/libpmemobj-cpp/build/tests/Release/cond_var.exe C:/projects/libpmemobj-cpp/build/test/cond_var_0_none/testfile
-- Stdout:
Signal 22, backtrace:
11: [00007FF72D716BE6]
10: [00007FF72D716D3B]
9: raise [00007FFC0A19DC17]
8: abort [00007FFC0A19EAA1]
7: [00007FF72D7128BD]
6: [00007FF72D71589D]
5: [00007FF72D712B77]
4: [00007FF72D712E06]
3: [00007FF72D712959]
2: o__realloc_base [00007FFC0A14CD70]
1: BaseThreadInitThunk [00007FFC11E313D2]
0: RtlUserThreadStart [00007FFC146E54F4]
End of stdout
-- Stderr:
C:\projects\libpmemobj-cpp\tests\cond_var\cond_var.cpp:178 reader_mutex_until - assertion failure: diff < epsilon
End of stderr
CMake Error at C:/projects/libpmemobj-cpp/tests/helpers.cmake:203 (message):
   C:/projects/libpmemobj-cpp/build/tests/Release/cond_var.exe C:/projects/libpmemobj-cpp/build/test/cond_var_0_none/testfile failed: 150
Call Stack (most recent call first):
  C:/projects/libpmemobj-cpp/tests/helpers.cmake:228 (execute_common)
  C:/projects/libpmemobj-cpp/tests/cond_var/cond_var_0.cmake:36 (execute)