Rollup merge of #48143 - nikomatsakis:termination_trait_in_tests, r=eddyb

Termination trait in tests

Support the `Termination` trait in unit tests (cc https://github.com/rust-lang/rust/issues/43301)

Also, a drive-by fix for #47075.

This is joint work with @bkchr.

src/libstd/lib.rs

@@ -500,11 +500,6 @@ mod memchr;
 // The runtime entry point and a few unstable public functions used by the
 // compiler
 pub mod rt;
-// The trait to support returning arbitrary types in the main function
-mod termination;
-
-#[unstable(feature = "termination_trait", issue = "43301")]
-pub use self::termination::Termination;
 
 // Include a number of private modules that exist solely to provide
 // the rustdoc documentation for primitive types. Using `include!`