test: limit npm test to test files

[johnjheejin]

Summary

• limit npm test discovery to test/**/*.test.mjs
• stop configuration examples named *_test.mjs from being counted as test files

Root cause

The bare node --test command used default discovery rules, which also matched four configuration examples under configs/.

User impact

Test output now reports only the actual automated test suite and no longer executes configuration examples as test modules.

Validation

• npm test (5 tests passed)
• node --check quarkify.mjs
• git diff --check