mk: Parameterize tests.mk to the max
mk: Don't check core/std/rustc as part of check-fast

When debugging check-fast it's annoying to wait through core/std
mk: Add output files to tests so they don't rerun unnecessarily
auto merge of #4814 : brson/rust/check, r=nikomatsakis

r? @nikomatsakis The third commit here is the one that makes the test suite remember which steps have completed successfully.