Prepare to move float tests from std to core

In order to facilitate cleaner git history, move the files in a separate
commit from the rest of the test changes.

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;