Move float tests from std to core

Many float-related tests in `std` only depend on `core`, so move the
tests there. This also allows us to verify functions from
`core_float_math`.

Since the majority of test files need to be moved to `coretests`, move
the files here without any cleanup; this is done in a followup commit.
This makes git history slightly cleaner, but coretests will not build
immediately after this commit.

Author: tgross35

library/std/tests/floats/f128.rs renamed to library/coretests/tests/floats/f128.rs

@@ -144,6 +144,7 @@ mod cmp;
 mod const_ptr;
 mod convert;
 mod ffi;
+mod floats;
 mod fmt;
 mod future;
 mod hash;