[libc][NFC] Add library of floating point test matchers.

This eliminates UnitTest's dependency on FPUtil and hence prevents
non-math tests from depending indirectly on FPUtil. The patch
essentially moves some of the existing pieces into a library of its own.

Along the way, renamed add_math_unittest to add_fp_unittest.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D85486