[libc] [NFC] Use matchers in tests

Summary:
Use `EXPECT_THAT` where possible in tests NFC intended.

Added a `void *` template instantiation of `StreamWrapper::operator<<`

Reviewers: sivachandra, PaulkaToast

Reviewed By: sivachandra

Subscribers: MaskRay, tschuett, libc-commits

Differential Revision: https://reviews.llvm.org/D75717