Removed outdated ui test suite README, give reasons for disabled tests

[jieyouxu]

Changes

• tests/ui/README.md is very outdated, test suite docs should consistently live in rustc-dev-guide.
• Add reasons for //@ ignore-test tests that don't have one.¹

This is a precursor change to make follow-up changes easier (possibly more specialized directives or converting some auxiliaries into the canonical auxiliary/ form).

Footnotes

1. searched via rg --no-ignore -F -e "ignore-test" tests/. ↩

[jieyouxu]

Hm, is triagebot down 🤔
@rustbot label +T-compiler +A-testsuite
@rustbot ready
r? compiler

[Zalathar]

Note that compiletest will also ignore any directory containing a file named compiletest-ignore-dir, which could be useful in some cases.

rust/src/tools/compiletest/src/lib.rs

Lines 762 to 765 in 1bc5618

	// Ignore directories that contain a file named `compiletest-ignore-dir`.
	if dir.join("compiletest-ignore-dir").exists() {
	return Ok(());
	}

[compiler-errors]

I think it's fine to mark precondition-checks files individually as unimplemented, b/c they make become implemented separately. Surprised we even have compiletest-ignore-dir 🤔 probably for different purposes than ignoring test failures (but instead, e.g., for file layout reasons).

@bors r+ rollup

[bors]

📌 Commit db6e3aa has been approved by compiler-errors

It is now in the queue for this repository.

[jieyouxu]

Oh I should've mentioned, I saw some of the precondition-check tests under the same dir active, which is why I didn't ignore that whole directory. (I thought the ignore dir comment was a general remark.)

See https://github.com/rust-lang/rust/tree/master/tests%2Fui%2Fprecondition-checks, only these few precondition-check tests (that is, the ones changed in this PR) aren't implemented yet.