[tests] Include private headers using relative paths

[bertmaher]

Description: While it's best for tests to only use the "public" headers of a library, it sometimes makes sense to use the private/internal headers directly. Instead of adding to the include path via cmake, let's refer to them using relative paths like ../../lib/Foo/Bar.h. This seems to be LLVM's approach, and I see two advantages:

1. It makes clear you're testing internal implementation, and
2. It plays nicely with FB's internal header layouts O:-)

Testing: ninja all

Documentation: n/a

[nadavrot]

This seems to be LLVM's approach, and I see two advantages:

@bertmaher Where is LLVM using the relative path include? I could not find this patten using grep.

[bertmaher]

A handful of unittests appear to use it, e.g. https://github.com/llvm-mirror/llvm/blob/master/unittests/CodeGen/DIEHashTest.cpp#L9. It's not very common.

Fwiw, I don't exactly love this approach. An alternative is moving these headers to include/glow (kind of awkward for the backend-specific headers). Or I could not do this, and instead add some hacks to the fb internal build system to look for headers in "unexpected" places. Or maybe there's something else I haven't thought of. This change just seemed like the least-bad approach.

[nadavrot]

@bertmaher Ah, okay, now I understand. Thank you.