libtest: utilize jobserver

[ishitatsuyuki]

Tests are normally CPU consuming tasks, and we can transparently utilize jobserver to achieve efficient core utilization.

Some usecases I can think of:

• Tests which may spawn threads (including rustc codegen-units, but not limited to)
• Using a parent make somewhere

[gilescope]

Not having this was the reason why Alex said that parallel test running PRs could not be accepted into cargo, so this would unblock parallel testing across multiple crates.