Move libXtest into libX/tests

This change moves:

1. `libcoretest` into `libcore/tests`
2. `libcollectionstest` into `libcollections/tests`

This is a follow-up to #39561.

src/libcore/num/bignum.rs

@@ -19,7 +19,7 @@
 //! inputs, but we don't do so to avoid the code bloat. Each bignum is still
 //! tracked for the actual usages, so it normally doesn't matter.
 
-// This module is only for dec2flt and flt2dec, and only public because of libcoretest.
+// This module is only for dec2flt and flt2dec, and only public because of coretests.
 // It is not intended to ever be stabilized.
 #![doc(hidden)]
 #![unstable(feature = "core_private_bignum",

src/libcore/num/diy_float.rs

@@ -10,7 +10,7 @@
 
 //! Extended precision "soft float", for internal use only.
 
-// This module is only for dec2flt and flt2dec, and only public because of libcoretest.
+// This module is only for dec2flt and flt2dec, and only public because of coretests.
 // It is not intended to ever be stabilized.
 #![doc(hidden)]
 #![unstable(feature = "core_private_diy_float",

src/libcore/num/flt2dec/mod.rs

@@ -118,7 +118,7 @@ provide a large enough buffer and `Part` array, and to assemble the final
 string from resulting `Part`s itself.
 
 All algorithms and formatting functions are accompanied by extensive tests
-in `coretest::num::flt2dec` module. It also shows how to use individual
+in `coretests::num::flt2dec` module. It also shows how to use individual
 functions.
 
 */

src/libcore/num/mod.rs

@@ -90,7 +90,7 @@ impl<T: fmt::UpperHex> fmt::UpperHex for Wrapping<T> {
 
 mod wrapping;
 
-// All these modules are technically private and only exposed for libcoretest:
+// All these modules are technically private and only exposed for coretests:
 pub mod flt2dec;
 pub mod dec2flt;
 pub mod bignum;