misc test cleanups

[cyrgani]

These are some mixed cleanups to tests/ui that individually seemed too small for a PR of their own. Some duplicated tests are removed, issues-* tests are renamed and more FIXMEs are added to ui/README.md.
Reasoning for the deleted tests:

• tests/ui/associated-types/issue-47814.rs: duplicate of tests/ui/associated-consts/issue-47814.rs
• rename tests/ui/closures/2229_closure_analysis/migrations/issue-78720.rs to .../nested-copy-drops-83176.rs since ICE: compiler/rustc_middle/src/ty/sty.rs:399:33: upvar_tys called before capture types are inferred #78720 was not the correct issue, ICE when using pattern and lambda with disjoint_capture_drop_reorder #83176 was.
• tests/ui/specialization/defaultimpl/specialization-feature-gate-default.rs: duplicate of tests/ui/specialization/specialization-feature-gate-default.rs
• fn/issue-1900.rs: duplicate of error-codes/E0131.rs

r? @Kivooeo

[rustbot]

This PR modifies tests/ui/issues/. If this PR is adding new tests to tests/ui/issues/,
please refrain from doing so, and instead add it to more descriptive subdirectories.

[Kivooeo]

@bors r+ rollup

[rust-bors]

📌 Commit 6cd2656 has been approved by Kivooeo

It is now in the queue for this repository.