testcase: give more time for instance destruction.

During teardown, we sometimes have errors that processes are still
running after being stopped, something like:
    
    Fault: <Fault 91: 'STILL_RUNNING: TestTelegraf-0'>

This is because we only retried `slapos node report` two times, but
`slapos node report` tells supervisor to stop processes without
waiting and when processes takes time to stop, this is sometimes not
enough. Increase to retry up to 10 times by default.

/reviewed-on nexedi/slapos.core!178

slapos/testing/testcase.py

@@ -379,7 +379,7 @@ class SlapOSInstanceTestCase(unittest.TestCase):
   # maximum retries for `slapos node instance`
   instance_max_retry = 10
   # maximum retries for `slapos node report`
-  report_max_retry = 2
+  report_max_retry = 10
   # number of partitions needed for this instance
   partition_count = 10
   # reference of the default requested partition