Move `check-bootstrap` from a makefile rule to `test::Bootstrap`

[jyn514]

It's very confusing that this can't actually be run through x.py itself; it tripped me up in #96687.

See

rust/src/bootstrap/mk/Makefile.in

Lines 48 to 49 in 1b2e0b6

	check-bootstrap:
	$(Q)$(CFG_PYTHON) $(CFG_SRC_DIR)src/bootstrap/bootstrap_test.py

for the existing code and

rust/src/bootstrap/test.rs

Lines 2349 to 2355 in 1b2e0b6

	impl Step for Bootstrap {
	type Output = ();
	const DEFAULT: bool = true;
	const ONLY_HOSTS: bool = true;
	/// Tests the build system itself.
	fn run(self, builder: &Builder<'_>) {

for where it should be moved.

@rustbot label +A-rustbuild +E-easy +E-mentor +E-help-wanted +A-contributor-roadblock

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