Composer script for PHPUnit coverage report

[JakeQZ]

I've read this, and still don't know how to get the report. I'm sure it's a simple command line option, but can't find easily find it.

TBH, I was barely aware this PHPUnit feature existed.

Unless I'm mistaken, we should add a Composer script (command) to run PHPUnit with whatever command line option is needed to generate the report. This will prompt developers on the project to have a look at it, perhaps initially out of idle curiosity, but potentially leading to improvements in the robustness of the unit tests.

[oliverklee]

Note for myself: https://github.com/eliashaeussler/cache-warmup/pull/327