test: three GC-rooting witnesses from #7962/#7978 are DARK — registered in no suite, run by nothing #8004

python3 scripts/check_test_registration.py currently reports three DARK TESTS
on main:

- test-files/test_gap_gc_container_value_rooting.ts          (added by #7962)
- test-files/test_gap_gc_define_properties_key_rooting.ts    (added by #7962)
- test-files/test_gap_gc_define_property_descriptor_rooting.ts (added by #7978)

Each exists on disk but is registered in no suite registry, so
scripts/gc_repsel_matrix.sh — and with it gc-stress,
gc-moving-witnesses and gc-ptr-shape-off-witness — never runs them.

test-parity/gc_repsel_corpus.txt's own header says exactly what this costs:

A NEW TEST FILE MUST BE REGISTERED HERE OR IT WILL NOT RUN. … An
unregistered witness is not a failing test, it is no test at all: the PR that
adds it goes green having run it zero times. That happened four times --
#7192, #7216, #7252 and #7270/#7271 -- before it was gated.

This is the fifth, sixth and seventh occurrence, and all three are GC-rooting
witnesses — precisely the class the matrix exists for. The three PRs that added
them were green having run them zero times under every GC arm.

Fix is one line each in test-parity/gc_repsel_corpus.txt, but registering them
newly runs three rooting witnesses under every matrix arm, which should be their
authors' call (@ the owners of #7949 / #7978) rather than a drive-by.

Related: scripts/check_test_registration.py runs in lint, a REQUIRED
context, and Tests has been failing on main for at least five consecutive
days (2026-08-08 .. 2026-08-12) — which is how three of these accumulated
unnoticed. The other red step in that job was check_thread_locals.py,
re-recorded in #7994.