persist-state: Clean up the temporary file upon destroy

When destroying a persist state, unlink the temporary file too, as to
not leave junk behind. Added test cases for both the commit() +
destroy() and the cancel() + destroy() cases. With this fixed, the
persist files created during make check need not be in CLEANFILES.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>