CI: improve the test running

[jrfnl]

For all sorts of reasons it would be good to have some more variation in the test environments on which the tests are run in CI.

Some suggestions/ideas:

• Run some test runs via PHPUnit installed via Composer, others with a PHPUnit PHAR so both autoloading mechanisms are tested.
• Run tests both against Ubuntu as well as Windows OS. Maybe also MacOS ?
• Add test runs with select PHP extensions disabled and potentially some functions disabled.
  Think:
  • Curl vs Fsockopen.
  • Decompression tries various PHP methods depending on availability. As the first one is available in the default CI setup, the logic and code for the fall-back methods is never actually tested.