[libc] Move FPExceptMatcher out of TestHelpers.h

TestHelpers.h pulls few pieces from LLVM libc's unittest framework
which aren't available on platforms like Fuchsia which use their own
unittest framework. So, by moving FPExceptMatcher to a different file
we can exclude LLVM libc specific pieces in a cleaner way.

In a later pass, it might make more sense to rename TestHelpers.h also
to FPMatcher.h. That way, we can make macros like EXPECT_FP_EQ to be
equivalent to EXPECT_EQ on platforms like Fuchsia.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D107129