Require (and potentially enforce) attribute-style `// issue: rust-lang/rust#12345` comments in issue-related tests

[tgross35]

This comes as a followup to #113583

The goal is to add meta comments to all tests related to an issue or ICE (e.g. regression tests) so that we can programmatically extract this information. For example:

// issue: rust-lang/rust#12345
// issue: rust-lang/rust#6789
// ice: rust-lang/rust#1234

// sample link to different repo:
// issue: rust-lang/cargo:1234   

We should make having this a goal for all regression tests going forward, and as a requirement with any renaming that might happen to suit #113583. We can likely do some of this programmatically.

[tgross35]

@rustbot label +A-testsuite +T-bootstrap -needs-triage

[workingjubilee]

We should probably allow naming issues that aren't in the main repo, so it should probably be more like {repo}#{issue}.

EDIT: Please disregard my inability to read more than 5 lines.
EDIT EDIT: The "real" reason to use repo#issue is that {org}/{repo}#issue is a shorthand in particular that GitHub understands and will "linkify" while browsing commit messages.

[matthiaskrgr]

Yeah we need a clear naming scheme for subtrees, for example when you add a test to clippy, inside the clippy repo, issue:123 looks fine because of course it refers to the clippy repo. But then it gets subtree-merged into the rustc repo and would point at some random rust-lang/rust ticket from 7 years ago (because the test is somewhere in src/tools/clippy inside the rustc repo now?)

[notriddle]

I've been using URLs because my editor lets me Ctrl+Click a URL to open it in a browser, while shorthand $org/$repo#$number only works in GitHub itself. Not sure if that's a big enough deal to justify using full URLs, though.