ci: Add EXTRA_TESTS custom pipeline variable

This variable can be set in scheduled pipelines and when manually starting pipelines. When set to openqa it causes the iso-installer-x86_64 and test-iso-installer-x86_64 jobs to run by default, providing a one-step method to trigger an OpenQA test cycle.

This is a big improvement on the current method which can take several hours and involves: (1) waiting for 'track' and 'build' to complete, (2) starting 'iso-installer-x86_64' manually, (3) waiting for that to complete, (4) finally, starting 'test-iso-installer-x86_64'.

Fixes #517 (closed)