[llvm] [libc] [libc][NFC] Split str_to_float tests and reduce type clutter (PR #75471)

[Guillaume Chatelet via llvm-commits]

gchatelet wrote:

> @michaelrj-google let me know if I implemented what you had in mind. We can iterate from here.

I'll submit to unblock other changes. Let me know if you want to change this and I'll do it in a subsequent patch.

https://github.com/llvm/llvm-project/pull/75471