Create integration tests

[withinboredom]

We currently build docker images in each PR but don't use them. We can, however, use those images and example repos like dunglas/frankenphp-drupal, dunglas/frankenphp-wordpress, etc. to spin up integration tests with "real world" code and verify (at least) a lack of seg-fault. The idea is to run it manually before merging (I imagine these tests will take a while to run).

Perhaps we can use something like dapr/mechanical-markdown to script it?