Testing mutants can be slower under nextest

[sourcefrog]

Nextest is faster at running passing test suites, but it's often slower at test suites that fail, at least if they have tests split across multiple test targets. And cargo-mutants generates many test suites that fail.

I think the best place to fix this is in Nextest, with an option to kill jobs once one test fails:

https://github.com/nextest-rs/nextest/discussions/2482