ci: merge test phase with build phase

This should speed up CI a bit.

CI stages are separated with a barrier, i.e. all the jobs from the
previous stage must finish before first job of a next phase starts.

In our current build phase we have the following jobs:
 * build (gcc)
 * build (clang-sanitizers)
 * clang-tidy

The build jobs are kind-of fast (ca. 5 and 10 mins, but not in parallel,
i.e. 15 mins in total) and the clang-tidy job is slow (ca. 27 mins).
After the clang-tidy jobs finishes we move on to the test phase where
the CI has to download dependencies (i.e. the build directory) in order
to run tests (ca. 3 + 5 mins, i.e. 8 minutes). I think we can squeeze
this 8 minute-test-jobs into the build phase where we have to wait
anyway.

Also, we don't have to upload and download the build directory which is
a big plus (and can save some time too).