[libc] [llvm] [libc][NFC] Reduce type clutter in str_to_float_test (PR #75353)

[via llvm-commits]

https://github.com/lntue commented:

Without the types it's actually a bit hard to see which type is being tested.  Does it make sense to split this test into several files: `str_to_float.h` containing the test template, then `str_to_float.cpp`, `str_to_double.cpp`, `str_to_long_double.cpp` or something like that for test specializations.

https://github.com/llvm/llvm-project/pull/75353