Define file extensions for expected and args files of Units test facility

Use .tags as extension for expected file. Use .ctags as extension for args
file. Write about args.ctags file in Units/README.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Merge pull request #51 from masatake/units-test-extension

Define file extensions for expected and args files of Units test facility