[Support/Path] Add more tests and improve failure messages of existing ones

Summary:
I am preparing a patch to the path function. While working on it, I
noticed that some of the areas are lacking test coverage (e.g. filename
and parent_path functions), so I add more tests to guard against
regressions there.

I have also found the failure messages hard to understand, so I rewrote
some existing test to give more actionable messages when they fail:
- for tests which run over multiple inputs, I use SCOPED_TRACE, to show
  which of the inputs caused the actual failure.
- for comparisons of vectors, I use gmock's container matchers, which
  will print out the full container contents (and the elements that
  differ) when they fail to match.

Reviewers: zturner, espindola

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45941

llvm-svn: 330691