libtest: provide a way to run *all* tests

[vext01]

I originally raised this against Cargo, but I've since learned that this is to do with libtest inside Rust itself.

So we have:

cargo test

To run all but ignored tests. Then:

cargo test -- --ignored

To run only ignored tests.

But as far as I can see there is no way to run all tests in parallel short of:

cargo test & cargo test -- --ignored

Which isn't ideal for scripting and is not convenient.

How about?

cargo test -- --all

Cheers

[Emerentius]

Exercism depends a lot on ignored-by-default tests so this would be really useful. A short look over libtest makes me believe that this would be very easy to add. I'll try my hand at it.

[eddyb]

Reopening because I had to revert #53527 to land edition-critical #54116.

[corneliusroemer]

@Emerentius I came here because I'm just working through Excercism and don't want to uncomment all the ignored tests ;)