Update test suite to future-proof base images

[clue]

This simple changeset updates the test suite to future-proof base images. Likewise, legacy HHVM is now executed in an up-to-date base image, but using a legacy container instead. This way, we no longer depend on any legacy base images that will be removed in the future as per https://github.blog/changelog/2022-08-09-github-actions-ubuntu-22-04-is-now-generally-available-on-github-hosted-runners/, https://github.blog/changelog/2022-08-09-github-actions-the-ubuntu-18-04-actions-runner-image-is-being-deprecated-and-will-be-removed-by-12-1-22/ and actions/runner-images#6002. I've confirmed pulling container images from Docker Hub does not appear to be rate-limited as per actions/runner-images#1445. Once these changes are merged, we should apply similar updates to all our other components.

Builds on top of #297 and #289

[clue]

Updated to work around failing test case for legacy HHVM (trying to allocate 1048576 file descriptors turns out to be a bad idea). This is now ready for review

[clue]

I've just updated this to use actions/checkout@v3 instead of v1 and only use the legacy HHVM container to run Composer and PHPUnit (also refs https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/ and https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/). This is now ready for review (again)

[WyriHaximus]

Thanks for the updates, looks even better now 👍 .