Intermittent test failures in `test.allocator` (MSVC, OMP host)

```
FAILURE: TestAllocatorCustomDestroy
[testing/allocator.cu:129] values are not equal: 1 0 [type='bool']
```

It happens about 1 in 8 times, but has only appeared in these configurations:

- host=omp device=cpp
- host=omp device=tbb

I have not been able to reproduce it on any other combination of host={cpp, tbb, omp} device={cpp, tbb, omp, cuda}. It has only happened in my MSVC 2019 builds and I have not been able to repro it on linux with GCC 7.5.0.