[CDASH] add option to set end time for ctest tests + slurm script for it

Finally found the right way to provide the stop time: without date and without timezone

My local test on cbmfles01 and vae23 with Experimental builds seem to work and stop as expected, we will probably have to wait the first runs to see how helpful the CDASH report is when the timeout is triggered.
It seems that the devs are still working on this: CMAKE issue 25146: CTest: incomplete report when stop-time is reached
But that should still be better than the current "no report at all", at least to identify areas of improvement on our test durations.