run community builds in CI

[staabm]

wdyt about running the test-suite of a few popular repositories in phpunit CI, so we can more easily which impact a given pull request has on them?

e.g. running the test-suite of symfony, phpstan or similar at a fixed commit (similar to how phpstan runs on several popular projects in CI).

such a build might also have indicated the problem we saw with https://github.com/sebastianbergmann/comparator/pull/119