Remove $DIR replacement of test's parent directory

[Alexendoo]

I've often wanted to ditch the $DIR replacement to make paths clickable in the terminal, this would allow that

[oli-obk]

Will that work in clippy and miri CI? If so, should we just ditch it in general?

[Alexendoo]

Yeah it should:tm: work fine as it's replacing a relative path. The biggest thing is it will be a hefty diff when upgrading ui_test

Ditching it in general sounds good to me, it could be recreated without a new Match by using the per test config callback if it's desired somewhere

[Alexendoo]

Also removed the RUSTLIB replacement as neither clippy or MIRI use it (tested in the rust repo as well)