fix(test): skip cache-tests when submodule is missing

[edenbuilds]

Summary

• npm test runs test:cache-tests, which imports test/fixtures/cache-tests/.../runner.mjs.
• That path is a git submodule. CITGM (and plain npm-pack installs) do not check it out, so the suite fails with Cannot find module '.../runner.mjs' (Test failure on CITGM for Node.js 26.6.0 #5642).
• Skip cleanly with a warning when the runner is absent — same idea as the WPT runner’s missing-checkout handling.

Fixes #5642

Test plan

• With empty test/fixtures/cache-tests (no submodule): node test/cache-interceptor/cache-tests.mjs --ci exits 0 and prints the skip warning
• With submodule initialized: cache-tests still run as before

Made with Cursor

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.45%. Comparing base (dd85997) to head (478a408).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5661      +/-   ##
==========================================
+ Coverage   93.43%   93.45%   +0.02%     
==========================================
  Files         110      110              
  Lines       38733    38776      +43     
==========================================
+ Hits        36190    36238      +48     
+ Misses       2543     2538       -5     

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[edenbuilds]

CI note: the Node 24/26 failures are in test/http2-request-never-settles.js (unrelated to this one-line cache-tests submodule guard). Re-running the failed jobs.

[mcollina]

lgtm

[edenbuilds]

Thanks @mcollina. Flagging that the red check is unrelated, so it doesn't hold this up by mistake.

Test and Coverage with Node.js 24 on ubuntu-latest reports 1505 passed, 1 failed, and the one failure is:

✖ failing tests:
test at test/http2-request-never-settles.js:1:1
✖ test/http2-request-never-settles.js (10130.007556ms)
  'test failed'

That's the HTTP/2 suite timing out at ~10.1s. This PR touches a single file, test/cache-interceptor/cache-tests.mjs, and only adds an early exit(0) when the test/fixtures/cache-tests submodule is absent — no runtime code and nothing in the h2 path. The branch is also already even with main (86b62998).

Looks like a timing-sensitive flake on a shared runner. Happy to rebase to trigger a fresh run if you'd prefer a green tick before merging — I held off since a force-push can drop the approval.

[edenbuilds]

Ready to merge from our side — @mcollina already approved.

The only red check is Test and Coverage with Node.js 24 on ubuntu-latest, failing in test/http2-request-never-settles.js (~10.1s timeout). This PR does not touch that path (one-line early-exit in test/cache-interceptor/cache-tests.mjs when the submodule is missing).

That same Node 24 job has also been failing on main recently, so it looks like a shared flake rather than a regression from this change. Happy to rebase again if useful — otherwise this should be safe to land.