./x.py test doesn't respect --json-output for rustc error messages

[bjorn3]

./x.py check doesn't check tests for the standard library, so I have to use ./x.py test instead when editing tests. Because --json-output is not respected for ./x.py test by bootstrap, I can't get inline error messages in my ide.

[onur-ozkan]

When you run x test, bootstrap runs formatting with rustfmt which doesn't support JSON outputs. As a workaround, you can run x test on a specific path to skip formatting so you can use the --json-output flag.

[bjorn3]

I was already doing ./x.py test library/alloc, but it still didn't show errors as json messages. AFAICT the issue is that bootstrap doesn't pass --message-format json for cargo test invocations.

[onur-ozkan]

That's another problem then, will check that tomorrow.

[onur-ozkan]

Should be fixed with 10f1238.