CI: Extend integration testing

Refactor the integration test script to support running a variety of tests, and add CI jobs for them.

We might want to combine some of these jobs, or otherwise adjust things, but this was minimal effort to add and should improve our test coverage a bit.

pmb/core/config.py

@@ -62,7 +62,7 @@ class Config:
     locale: str = "en_US.UTF-8"
     mirrors: Mirrors = {
         "alpine_custom": "none",
-        "alpine": "http://dl-cdn.alpinelinux.org/alpine/",
+        "alpine": "http://alpine.mirror.far.fi/",
         "pmaports_custom": "none",
         "pmaports": "http://mirror.postmarketos.org/postmarketos/",
         "systemd_custom": "none",