fix: initialize selected submodules for Undici CITGM

[mcollina]

Summary

Fix the Undici CITGM failure reported in nodejs/undici#5642.

CITGM's Undici lookup entry runs test:cache-tests, but the normal GitHub source archive does not contain the contents of the test/fixtures/cache-tests submodule. Use a shallow Git clone and initialize only that required submodule. Avoid initializing the WPT submodule, which is unnecessarily large and is not run by this entry.

This adds a submodules lookup option for selecting submodules when useGitClone is enabled.

Tests

• npm test
• End-to-end selective clone verified the cache-tests runner is present and WPT remains uninitialized.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.33%. Comparing base (ff3ea4a) to head (8dedb7e).

Files with missing lines	Patch %	Lines
lib/git-clone.js	55.55%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1115      +/-   ##
==========================================
- Coverage   96.50%   96.33%   -0.17%     
==========================================
  Files          29       29              
  Lines        2203     2213      +10     
==========================================
+ Hits         2126     2132       +6     
- Misses         77       81       +4     

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