Read the room, skip the rest

[cds-amal]

The UI tests (where we run stable-mir-json against rustc's own test suite) have a versioning problem that mirrors the golden file problem solved in PR #144: different nightlies have different sets of UI tests. Files get added, deleted, and renamed between nightly commits. A test that exists in nightly-2025-03-01 might be gone by nightly-2025-10-03, or moved to a different directory. Running a stale test list against a newer nightly produces spurious failures; maintaining the lists by hand is tedious and error-prone.

This PR adds the tooling to generate, validate, and run per-nightly UI test lists automatically. Three pieces:

• parse_test_directives.awk: an awk script that extracts //@ directives from rustc test source files and decides whether a test should be skipped on the current host. It handles only-<target>, ignore-<target>, needs-sanitizer, needs-subprocess-spawning, edition directives, compile-flags, and a handful of environment-specific skips. A "universal" mode suppresses platform-specific filtering so that generated lists are correct on any host; platform filtering happens at runtime instead.

One environment-specific skip worth calling out: tests that reference extern crate libc are skipped because our sysroot contains both .rmeta and .rlib artifacts for libc. Rustc, invoked directly outside cargo, sees two candidates for the same crate and bails with E0464. Cargo normally sidesteps this by passing --extern libc=/exact/path, but we don't have that luxury. The skip is a pragmatic workaround, not a judgment on the tests themselves.

• diff_test_lists.sh: given a rust-lang/rust checkout, this script diffs the tests/ui/ directory between the base nightly commit and a target nightly commit. It tracks file deletions, renames, and additions, then applies them to the base passing.tsv/failing.tsv to produce effective per-nightly test lists. The output is deterministic: same repo + same commits = same lists. Supports --report (human-readable diff summary), --emit (write lists to tests/ui/overrides//), and --chain (show incremental diffs between consecutive nightlies).

• Rewrites of run_ui_tests.sh and remake_ui_tests.sh: both now use the shared directive parser instead of inline awk snippets, pick up per-nightly override lists when available, and handle architecture filtering correctly. run_ui_tests.sh also fixes the RUN_SMIR library path issue that caused failures on some setups.

The PR includes pre-generated override lists for all 13 supported nightlies (2025-03-01 through 2026-01-15), a unit test suite for the directive parser (test_directives_test.sh, ~420 lines of boundary-condition tests), and corresponding Makefile targets (make test-ui, make test-ui-emit, make test-directives).

Test plan

• make test-directives passes (unit tests for the awk parser)
• make test-ui RUST_DIR_ROOT=/path/to/rust passes with the pinned nightly
• make test-ui-emit RUST_DIR_ROOT=/path/to/rust NIGHTLY=nightly-2025-03-01 generates lists matching the checked-in overrides

[cds-amal]

I'm open to paring this down :) Solaris, it's been a while.

[cds-amal]

A note to reviewers, pay attention to the parse_test_directives.awk.