This adds a new test fixture class FEnvSafeTest (usable as a base
class for other fixtures) that ensures each test doesn't perturb
the `fenv_t` state that the next test will start with.  It also
provides types and methods tests can use to explicitly wrap code
under test either to check that it doesn't perturb the state or
to save and restore the state around particular test code.

All the fenv and math tests are updated to use this so that none
can affect another.  Expectations that code under test and/or
tests themselves don't perturb state can be added later.

Hello, this addresses #87863.

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

This part of the effort to make all test related pieces into the `test`
directory. This helps is excluding test related pieces in a straight
forward manner if LLVM_INCLUDE_TESTS is OFF. Future patches will also move
the MPFR wrapper and testutils into the 'test' directory.

Differential Revision: https://reviews.llvm.org/D129619

Summary:
Having a consistent prefix makes selecting all of the llvm libc tests
easier on any platform that is also using the gtest framework.
This also modifies the TEST and TEST_F macros to enforce this change
moving forward.

Reviewers: sivachandra

Subscribers:

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D92546