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.

library/coretests/Cargo.toml

@@ -26,3 +26,14 @@ test = true
 [dev-dependencies]
 rand = { version = "0.9.0", default-features = false }
 rand_xorshift = { version = "0.4.0", default-features = false }
+
+[lints.rust.unexpected_cfgs]
+level = "warn"
+check-cfg = [
+    # Internal features aren't marked known config by default, we use these to
+    # gate tests.
+    'cfg(target_has_reliable_f16)',
+    'cfg(target_has_reliable_f16_math)',
+    'cfg(target_has_reliable_f128)',
+    'cfg(target_has_reliable_f128_math)',
+]

library/coretests/tests/lib.rs

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