always run test-selection stage

Which issue this MR fixes:

The GitLab CI sometimes seems to fail passing the created .yml file to all jobs of the trigger stage (some jobs randomly fail with invalid yml error message although all jobs receive the same file).

What this MR does / why does DuMux need it:

This MR moves the test-selection stage into the child pipelines, always executing it independent of the pipeline source. for non-merge-requests, the selection job exits directly and subsequent jobs build and test all tests.

Moreover, dunecontrol is run separately within the test selection and the build stage, because the test selection script relies on gcc being used (it uses gcc specific flags). This type of behavior had been erroneously changed with 53bba82c.

Fixes #968 (closed)