Option to run alternative or additional test commands

[sourcefrog]

In some trees it would be helpful to configure an additional or alternative test command rather than cargo test.

Possibly there should be an option to not run cargo test at all, in cases where the shell command is enough?

This probably isn't too hard, at least assuming that a fixed shell command is sufficient. cargo-mutants has some logic to compose cargo test commands but perhaps that's just not needed for external tests.

[Deadairx]

I'm taking a stab at this in my free time, I'll throw up a PR when I have something working :)