Evaluate using proper loopback requests for runtime checks

[swissspidy]

The runtime checks right now are not really smart, they just modify the global WP_Query to "navigate to" specific URLs and then see what kind resources were enqueued/inserted etc. That will never be as accurate as an actual HTTP request to the frontend.

This won't be very straightforward though, because for the runtime checks PCP spins up a separate WordPress install that doesn't affect the main site. So the system under test (SUT) won't actually be accessible via loopback request out of the box. But there might be some ways to address that, for example with some special query parameters or so.

It could also be more feasible for cases like the GitHub Action with a more closed down environment.