testing/testcase: don't destroy instance if it was not requested

This should prevent such confusing tracebacks when building software fail:

    INFO Finished software releases.
    ERROR:slapos.testing.testcase:Error during request destruction
    Traceback (most recent call last):
      File "parts/slapos.core-repository/slapos/testing/testcase.py", line 615, in _cleanup
	cls.requestDefaultInstance(state='destroyed')
      File "parts/slapos.core-repository/slapos/testing/testcase.py", line 680, in requestDefaultInstance
	cls._instance_parameter_dict)
    AttributeError: type object 'SlapOSInstanceTestCase for ...' has no attribute '_instance_parameter_dict'

slapos/testing/testcase.py

@@ -611,6 +611,7 @@ class SlapOSInstanceTestCase(unittest.TestCase):
     Catches and log all exceptions and take snapshot named `snapshot_name` + the failing step.
     """
     try:
+      if hasattr(cls, '_instance_parameter_dict'):
         cls.requestDefaultInstance(state='destroyed')
     except:
       cls.logger.exception("Error during request destruction")