test_runner: support mocking non-existent modules

[geeksilva97]

Fixes #55891

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[ljharb]

if it does exist on disk, though, it could be imported/required via other means, so it seems prudent to ensure the actual resolved specifier is still mocked out?

[geeksilva97]

That's an edge case I could not think of. Would you have an example to illustrate the issue?

[ljharb]

Let's say you have an exports specifier "foo": "./foo/index.js" in package pkg.

That can be required, or imported, by any of the following paths relative to the package root:

• pkg/foo
• ./foo/index.js
• ./foo (if it's a CJS file)
• ./foo/index (if it's a CJS file)

If you don't fully resolve to a list of potential paths and confirm that, in fact, nothing exists in any of them, then shadowing a subset of them would create a scenario where you have more than one source of truth about that resource's contents.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.69%. Comparing base (68d7b6f) to head (c1881e2).
⚠️ Report is 338 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62298      +/-   ##
==========================================
- Coverage   89.77%   89.69%   -0.09%     
==========================================
  Files         673      676       +3     
  Lines      203820   206657    +2837     
  Branches    39175    39575     +400     
==========================================
+ Hits       182987   185351    +2364     
- Misses      13152    13446     +294     
- Partials     7681     7860     +179     

Files with missing lines	Coverage Δ
lib/internal/test_runner/mock/loader.js	99.42% <100.00%> (+0.07%)	⬆️
lib/internal/test_runner/mock/mock.js	98.80% <100.00%> (+0.07%)	⬆️

... and 243 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.