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 (a6ce851).
⚠️ 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    36239      +49     
+ Misses       2543     2537       -6     

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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.