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

[Guillaume Chatelet via llvm-commits]

gchatelet wrote:

@michaelrj-google my original goal was to remove `LIBC_NAMESPACE` and `typename LIBC_NAMESPACE::fputil::FPBits<T>::UIntType` but I had to template the test class.
As such, it is less clear which type is being tested, let me know if you prefer to keep the type explicit.

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