Reorganize pipeline steps for compilation

[sebastian-peter]

Currently, building the project and the execution of tests is both happening within the same step. If the pipeline fails, this makes it hard to distinguish at one glance whether the failure occurred during building or tests.
For this part of the task, it's important to understand the difference between gradle assemble and gradle build and to get a grasp of how gradle works in general. (checkScoverage can probably removed, as that's done by sonarqube later)

At the same time, "sonarqube analysis" and "quality gate" are connected very closely and don't really need separate pipeline steps. If an error occurs here, it's clear that it's related to sonarqube in some way.