Restructure CI tests so they don't fail, add DDSim and Storage elements tests (SE disabled for now), add a unit tests

Jan Hendrik Ebbing requested to merge jebbing/ILCDIRAC:ddsim_fix into Rel-v25r0

CI config is changed to run all tests sequentially instead of parallel to avoid randomly failing. SETests are disabled until the bug with CASTOR/EOS third party copy is fixed.

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