Use Catch2 from LCG

Catch2 is available in the dev3 and dev4 LCG builds: https://its.cern.ch/jira/browse/LBCORE-1988

We need to prepare a MR that switches to using that and test it in the lhcb-lcg-dev3 lhcb-lcg-dev4 slots.

Once LBCORE-2041 is resolved, we can adapt and apply the MR to master.