Auto merge of #32348 - brson:cargotest, r=alexcrichton

Introduce 'cargotest' and the check-cargotest buildstep

This is a new suite of tests that verifies that the compiler builds specific revisions of select crates from crates.io.

It does not run by default. It is intended that bors runs these tests against all PRs, and gates on them. In this way we will make it harder still to break important swaths of the ecosystem, even on nightly.

This is a very basic implementation intended for feedback. The biggest thing it probably should do but doesn't is use a lockfile for every project it builds.

r? @alexcrichton cc @rust-lang/lang @rust-lang/libs