Cleanup: Rid the `rmake` test runners of `extern crate run_make_support;`
`run_make_support` is part of the *extern prelude* of `rmake` test runners rendering `extern crate run_make_support` redundant:
80451a485b/src/tools/compiletest/src/runtest.rs (L3826-L3827)
~~Contains some fmt'ing changes because I've enabled format-on-save in my editor and because we don't run `x fmt` for `rmake` test runners yet (this gets addressed by #124613). I can revert those if you'd like me to.~~ (reverted)
r? jieyouxu or testing-devex(?) or boostrap(?)
6cc3959230