CI testbed

[dfuchsgruber]

This issue serves to track CI test cases we should include to seml.

• Source Code Stashing: Test that the experiment actually uses the stashed source code. Proposal: Add experiments, then change the source code to include a raise RuntimeError(...) statement, then run the code and check that all experiments are COMPLETED eventually.
• Test that experiments_per_job actually runs code in parallel: Devise a python script with wait times as dummy and ensure that they start roughly synchronised by checking their outputs.
• Ensure that the src layout is correctly supported by source code stashing (how?)

Add further test cases when you encounter weird behaviour that needs to be fixed.