Rollup merge of #134423 - jieyouxu:bootstrap-test-valid, r=onur-ozkan

bootstrap: use specific-purpose ui test path for `test_valid` self-test

I wanted to move some ui tests around in #134418, which broke `test_valid` since it was referencing two non-specific-purpose ui tests. This PR instead adds two dummy tests under `tests/ui/bootstrap/self-test/`, for that purpose specifically.

r? bootstrap

tests/ui/bootstrap/self-test/a.rs

@@ -0,0 +1,2 @@
+//! Not used by compiler, this is used by bootstrap cli self-test.
+//@ ignore-test

tests/ui/bootstrap/self-test/b.rs

@@ -0,0 +1,2 @@
+//! Not used by compiler, used by bootstrap cli self-test.
+//@ ignore-test