tests: additional cases in test-unit-file

      These tests will be useful to check the cases regarding quoted and
escaped semicolon when we switch to using unquote_first_word.

Additionally, convert some of the tests that have semicolons so that the
argument after the semicolon looks like a path (starts with /) so that
we can see the change of behavior when making config_parse_exec more
strict about what it accepts as a command separator.