Run garbage collector during probe test setUp

DatabaseBrokers cache opened connections.  If a probe test
instantiates a DatabaseBroker, or any other class that in turn
instantiates a DatabaseBroker, such as a ContainerSharder, then
connections may hold db files open until the DatabaseBroker is garbage
collected. This can cause subsequent probe tests to fail during their
setUp() because resetswift is unable to unmount device directories
while db files are open.

A call to gc.collect() is added during setUp() to ensure db files are
closed before resetswift() is called.

Closes-Bug: 1917050
Change-Id: Ifda4407c9ecff4c636fe07e013c3ebcebd0df018

test/probe/common.py

@@ -16,6 +16,7 @@
 from __future__ import print_function
 
 import errno
+import gc
 import json
 import os
 from subprocess import Popen, PIPE
@@ -371,6 +372,10 @@ class ProbeTest(unittest.TestCase):
                 self.configs['proxy-server'] = conf
 
     def setUp(self):
+        # previous test may have left DatabaseBroker instances in garbage with
+        # open connections to db files which will prevent unmounting devices in
+        # resetswift, so collect garbage now
+        gc.collect()
         resetswift()
         kill_orphans()
         self._load_rings_and_configs()