test: use 'exit 0' instead of 'return' in test scripts

test: use 'exit 0' instead of 'return' in test scripts

14385s [   66.896852] TEST-87-AUX-UTILS-VM.sh[3744]: + test -x /usr/lib/systemd/systemd-validatefs
14385s [   66.898544] TEST-87-AUX-UTILS-VM.sh[3744]: + echo 'no systemd-validatefs'
14385s [   66.899115] TEST-87-AUX-UTILS-VM.sh[3744]: no systemd-validatefs
14385s [   66.899699] TEST-87-AUX-UTILS-VM.sh[3744]: + return
14385s [   66.900189] TEST-87-AUX-UTILS-VM.sh[3744]: .//usr/lib/systemd/tests/testdata/units/TEST-87-AUX-UTILS-VM.validatefs.sh: line 13: return: can only `return' from a function or sourced script

Follow-up for 6e003d18bbf5e9f55c08b25d80b138b3a98d5f26