Rollup merge of #125273 - onur-ozkan:bootstrap-self-test, r=albertlarsan68

bootstrap: implement new feature `bootstrap-self-test`

Some of the bootstrap logics should be ignored during unit tests because they either make the tests take longer or cause them to fail. Therefore we need to be able to exclude them from the bootstrap when it's called by unit tests. This change introduces a new feature called `bootstrap-self-test`, which is enabled on bootstrap unit tests by default. This allows us to keep the logic separate between compiler builds and bootstrap tests without needing messy workarounds (like checking if target names match those in the unit tests).

Also, resolves rust-lang/rust#122090 (without having to create separate modules)

Author: workingjubilee