Apparently Windows' .lib files are also ar. Add tests.

Change-Id: Ie35f410268086b8fe6d4d1b491de3f30a46309dd
Reviewed-on: https://boringssl-review.googlesource.com/c/33348
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>