Auto merge of #2398 - sunshowers:nextest-compat, r=RalfJung

[cargo-miri] support nextest

Add the ability to run `cargo miri nextest list` and `cargo miri nextest run`.

[cargo-nextest](https://nexte.st) is a new test runner for Rust maintained mostly by myself. It has several new features, but the most relevant to miri is the fact that it runs [each test in its own process](https://nexte.st/book/how-it-works.html#the-nextest-model). This gives miri users better leak detection (https://github.com/rust-lang/miri/issues/1481) for free, for example.

See https://github.com/nextest-rs/nextest/issues/181 for discussion, including comments by `@eddyb` and `@RalfJung.`

Future work might be to have miri read [the list of tests](https://docs.rs/nextest-metadata/latest/nextest_metadata/struct.TestListSummary.html) (or [test binaries](https://docs.rs/nextest-metadata/latest/nextest_metadata/struct.BinaryListSummary.html)) generated by `nextest list`. `@eddyb` thinks that might be useful.

I tested `cargo miri nextest run` against smallvec, and it worked great.

Note: Running tests out of archives is currently broken, as the comment in run-test.py explains.