Support different backend for testing

[farodin91]

I like how the test works in rust, but I want to integrate them into intellij-rust/intellij-rust#1485. Therefore it would be cool to get test results in a different format like JSON.

This could be done by each test returns a single line JSON object and a single line conclusion at the end.

[mkaput]

For IntelliJ Rust, actually TeamCity Service Messages would be perfect :)

[gilescope]

Yep, agreed. I like how NUnit3 does it. They have a '-teamcity' command line flag that turns on the service messages. This would make integration with enterprise teamcity instances a doddle. Should be really quite easy functionallity to add and for the utility it brings it seems a no-brainer for me. (I suspect TeamCity isn't the only CI that interprets the TeamCity Service Messages).

[gilescope]

Should have a PR ready for review in a few days to support the TeamCity service messages. Need to test it with TeamCity first and ensure there's tests around it.

[dtolnay]

I would love to support this but as discussed in #44813 let's track this as part of rust-lang/rfcs#816.