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