[llvm-strings] Improve testing of llvm-strings

This patch tidies up the llvm-strings testing by:

1. Adding comments to every test.
2. Getting rid of canned input files, and having the tests generate
   them on the fly (this makes the tests self-contained).
3. Adding missing test coverage.
4. Renaming some tests that weren't clear as to their purpose.
5. Adding extra checking of various cases, formatting etc.
6. Removing a test that didn't seem to have any useful purpose for
   testing llvm-strings.

Reviewed by: rupprecht, grimar, MaskRay

Differential Revision: https://reviews.llvm.org/D66015

llvm-svn: 368555