In general, it's better to avoid a negation. And "!" is special, because it is
used for history expansion, i.e. the same command would behave differently if
pasted on the command line.
Inspired by 4a899c5a23.
Rename TEST-23-TYPE-EXEC to TEST-23-UNIT-FILE and merge it with
following tests:
- TEST-37-RUNTIMEDIRECTORYPRESERV
- TEST-40-EXEC-COMMAND-EX
- TEST-41-ONESHOT-RESTART
- TEST-42-EXECSTOPPOST
- TEST-57-ONSUCCESS-UPHOLD