Optimize `tests.yml`

[yamcodes]

We should optimize tests.yml, as we recently had a PR (#233) where commits were made in succession (2-3 minutes apart). Since there was already a tests.yml action running, the consequent runs were already active before the first one saved back to the cache. This meant that all runs were running fully without addressing a cache, even though there were no relevant test source changes.

Possible solutions include concurrency, max-parallel in matrices, and using a container to cache the browsers: microsoft/playwright#7249 (comment)