Reduce test duration by avoiding compilation overhead

[rockdaboot]

Currently, several buildkite tests like TestProcessAllSettings are

• starting 8 docker containers (based on alpine) in sequence
• compiling the test code in each container

#243 tested running the 8 sub-tests of TestProcessAllSettings in parallel.
It shaved off ~1 minute from the 5 minutes, so maybe it's not worth the effort.
From my observations, I'd say the biggest bottleneck is the parallel compilation of the test code in 8 containers.

So, why not build a docker image with the compiled test code before, then running the tests in parallel. That should decrease the current 5min duration to an estimated ~45s.