Skipped test should be forbidden

[fcollonval]

Description

In #745, the configuration to forbid only and pending tests on CI was added. This successfully prevent the use of .only on CI. But actually does not forbid .skip as seen in the CI log of #745: https://github.com/jupyterlab/lumino/actions/runs/13718590093/job/38368769283

Relevant CI log for posterity

> @lumino/widgets:"test:chromium"

@lumino/[email protected] test
npm run build:test && web-test-runner tests/lib/bundle.test.js --node-resolve --playwright --browsers chromium

@lumino/[email protected] build:test
npm run clean:test && tsc --build tests && cd tests && rollup -c

@lumino/[email protected] clean:test
rimraf tests/lib tests/tsconfig.tsbuildinfo

./lib/index.spec.js → ./lib/bundle.test.js...
(!) Circular dependencies
../../../node_modules/chai/lib/chai.js -> ../../../node_modules/chai/lib/chai/utils/index.js -> ../../../node_modules/chai/lib/chai/utils/addProperty.js -> ../../../node_modules/chai/lib/chai.js
../../../node_modules/chai/lib/chai.js -> ../../../node_modules/chai/lib/chai/utils/index.js -> ../../../node_modules/chai/lib/chai/utils/addMethod.js -> ../../../node_modules/chai/lib/chai.js
../../../node_modules/chai/lib/chai.js -> ../../../node_modules/chai/lib/chai/utils/index.js -> ../../../node_modules/chai/lib/chai/utils/overwriteProperty.js -> ../../../node_modules/chai/lib/chai.js
...and 3 more
created ./lib/bundle.test.js in 1.9s

Chromium: |██████████████████████████████| 0/1 test files | 0 passed, 0 failed

Running tests...

Running 1 test files...

Chromium: |██████████████████████████████| 1/1 test files | 938 passed, 0 failed, 1 skipped


Reproduce

1. Flag a test with it.skip
2. Set environment variable CI=1
3. Execute the tests, it does not fail

Expected behavior

When on CI, skipped tests should result in failing tests.

Important

The test do failed when run in debug mode within the browser.