test-execute: check if private directories have bad permissions before running test_e...

test-execute: check if private directories have bad permissions before running test_exec_dynamicuser()

If the directory (/var/lib/private is most likely) has borked permissions, the
test will fail with a cryptic message and EXIT_STATE_DIRECTORY or similar. The
message from the child with more details gets lost somewhere. Let's avoid running
the test in that case and provide a simple error message instead.

E.g. systemd-238-12.git07f8cd5.fc28.ppc64 (which I encountered on a test machine)
has /var/lib/private with 0755.