twister: PyTest plugin's DUT release sequence change

Change Twister PyTest plugin's test finalizing sequence to release
the DUT it is used as the very last operation, after the Test Instance
status becomes fully updated from the execution results.

This also fix a race condition possible when pytest plugin releases
the DUT and it becomes acquired by another test while the current test
is not yet finalized completely.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>