This MR adds the nopost.sh post-script found in the master branch (https://gitlab.cern.ch/atlas/athena/-/blob/master/AtlasTest/TestTools/share/nopost.sh) to 21.2, in order to allow standalone unit tests without a ref file for packages built against the 21.2-series analysis releases.
See https://twiki.cern.ch/twiki/bin/view/AtlasComputing/SoftwareDevelopmentWorkBookCMakeInAtlas#atlas_add_test for documentation and intended use.