compiletest: avoid invoking rustc with absolute paths.

[eddyb]

This PR makes the compiletests runner change its current working dir to tests/compiletests/, allowing the source paths, that the compiletest crate invokes rustc with, to always be relative e.g. ui/foo/bar.rs.

Among other things, this should help with @Firestar99 not being able to get the ui/spirv-attr/invalid-target.rs test to pass, because the JSON output is slightly too long and gets abbreviated with a big <<<<<< SKIPPED 1026 BYTES >>>>>> in the middle, due to harcoded limits in the compiletest crate itself.

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This also cleans up the $OPSTRING_FILENAME hack used by disassembly tests, but I'm worried that will break on Windows again, so I'm opening this as a draft to see what that situation is like.
_{(I do think that the main reason for that hack was that the replacement of the test base path w/ $DIR wouldn't kick in on Windows, but that's a much bigger issue for absolute paths, than for relative ones, so it might be easier to work around it now)}

EDIT: the situation is predictable enough that I added a "ui/dis/" -> "$DIR" normalization rule to the impacted tests (by that point, compiletest had already replaced all \\ with /, so even if its own $DIR replacement failed, it should at least predictably result in paths like ui/dis/ even on Windows).

[Firestar99]

Works!

Eventually we should maybe have a default set of normalize-stderr-test that just applies to all tests, instead of pasting them everywhere.