Enable the regalloc checker in testsuites

👋 Hey,

This was brought up yesterday in the cranelift meeting, and I'm filing this mostly so we don't forget.

#### Feature

Enable the cranelift `regalloc_checker` flag in our test suites.

The concrete testsuites I'm thinking are the `filetests` testsuite for cranelift, and the `spec_testsuite` for wasm. However if we can get others then that would be great too.

#### Benefit

This would have caught #4969 earlier and potentially other regalloc issues in the future.

#### Implementation

I'm most familiar with `filetests` and at least in the `compile` and `run` tests we can "clone" the flags requested by the tests and force enable `regalloc_checker`. I'm not too sure about `spec_testsuite` but maybe something similar would work too?

Something that was brought up at the meeting was the performance of `regalloc_checker`, we should probably make sure that the testsuites don't become too slow.

#### Alternatives

#### Make it a default flag

@cfallin mentioned this yesterday with the additional note that the regalloc checker was never built for speed so it would probably have a somewhat large downside. This matches what I've seen when trying to fuzz it.

#### Run it only during fuzzing

I played around with this in the `cranelift-fuzzgen` fuzzer, and the preliminary results aren't great.

In the sample run that I usually do (~90k inputs), with regalloc checker we run at 10 execs / sec and without we get 26 execs / sec. The other alternative that I've been thinking is running with regalloc checker only x% of the time although that is still a WIP.

cc: @cfallin @jameysharp @uweigand