Test: Allow harnesses to be one-shot instead of requiring daemons.

Daemons are more efficient, but require more work to code up.  This
allows you to start with a one-shot harness and then add a daemon mode
later if necessary.

Used this to add the "cks" command-line conversion tool to the test
suite.  Should switch over to daemon mode at some point since launching
the tool every time is really slow.