Replace Appveyor and Travis with Github actions

Travis has recently introduced built limits which makes using them very
slow sometimes and I never really liked the setup for appveyor for our
Windows

Replace Appveyor and Travis with Github actions

Travis has recently introduced built limits which makes using them very
slow sometimes and I never really liked the setup for appveyor for our
Windows testing.

This makes use of github actions instead. Some notes:

* since we only support PHP 7.2+ anymore, nothing below is tested
* MySQL and Postgres auth backends are only tested on Linux (as before)
* I am not actually sure the problem matchers are working as intended
* There is currently no good way to run tests that are allowed to fail
so no PHP 8 tests are run yet
* I haven't reestablished the no-gzip test runs again, I'm not sure they
are still relevant on modern PHP systems
* PHP 7.4 currently fails because of https://github.com/MarcusSchwarz/lesserphp/pull/18
* I'm currently using @v2 tags for the used action. We had a discussion
before if we should actually pin the action to a fixed commit. For
sake of easier maintainance, I would argue to trust the setup-php
action.

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added a postgres example database to the tests

first go at using real database dumps for tests

this configures a minimal wordpress setup