[XRay][compiler-rt] Use explicit comparisons in unit tests.

Summary:
This should improve the error messages generated providing a bit more
information when the failures are printed out. One example of a
contrived error looks like:

```
Expected: (Buffers.getBuffer(Buf)) != (std::error_code()), actual:
system:0 vs system:0
```

Because we're using error codes, the default printing gets us more
useful information in case of failure.

This is a follow-up on D26232.

Reviewers: rSerge

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D27495

llvm-svn: 289501