chore(test): register test_gate_common.py as a ctest

[shipyard-local]

gate_common.py is the substrate every repo gate imports — trailer
collection, range diffing, glob matching. Its test file runs nowhere.
tools/scripts/test_*.py are not globbed into CI; coverage comes from
explicit, hand-maintained lists, and this file is in none of them.

test_gates.py does re-export some of this module's TestCase classes, so
part of it has been running indirectly. That mechanism is an explicit
import list, though, which makes the coverage silently partial: a class
added to test_gate_common.py is invisible to CI until someone remembers
to add it to test_gates.py's imports too. #6400 adds
SquashBuriedBypassTrailerTests without that update, so its tests do not
run. Registering the file directly runs whatever it contains, and holds
regardless of the import list.

Verified green in both orderings: 27 tests pass against gate_common.py
as it stands on main, and 31 pass against #6400's version, so this is
safe to land before or after that change.

One add_test entry. No behavior change.

---

🔎 Provenance

Agent	claude
Machine	m3
Tab	Timeline v3
Directory	/Volumes/Workshop/Code/pulp-ctest-reg
Session	c66f4a64-25ea-42e5-b184-d48611a066a2

Resume

claude --resume c66f4a64-25ea-42e5-b184-d48611a066a2

Jump to this tab

cmux surface focus C0B3DA54-39AF-4DAA-8089-AA08DAB71968

Relaunch (any agent)

cmux surface resume get --surface C0B3DA54-39AF-4DAA-8089-AA08DAB71968

Restore URL — https://claude.ai/code/session_01TVQHcnbg3t6TvXsdv6gXhS

_{stamped 2026-07-20 21:52 UTC}