Large test suites sometimes provide limited benefit to incremental development

[mbrodesser-Igalia]

If a test suite contains many (e.g. 100) tests and the first test fails in a way that the following tests aren't executed, incremental development can't make use them.

A recent example of this is https://searchfox.org/mozilla-central/source/testing/web-platform/tests/html/semantics/popovers/popover-light-dismiss.html.

One mitigation for this would be to split such files into multiple files, containing, ideally, just one test per file. If this is feasible, it should be explicitly mentioned at https://web-platform-tests.org/test-suite-design.html and reviewers should pay attention to it.