Clean up after ImportModulePoisonFixture

ImportModulePoisonFixture installs a MetaPathFinder to sys.meta_path to
catch unsuitable imports during testing. But the extra finder
wasn't removed during fixture cleanup so such finder is leaked between
test cases running in the same test executor. As all our test cases has
this poison set up by default this only caused unnecessary finders
accumulating in sys.meta_path.

Change-Id: I2692552000687d51158dad0d938d68e891b40d8a

Author: gibizer

nova/tests/fixtures/nova.py

@@ -1832,14 +1832,16 @@ def __init__(self, module_names):
         self.module_names = module_names
         self.fail_message = ''
         if isinstance(module_names, str):
-            self.module_names = set([module_names])
-        sys.meta_path.insert(0, self.ForbiddenModules(self, self.module_names))
+            self.module_names = {module_names}
+        self.meta_path_finder = self.ForbiddenModules(self, self.module_names)
 
     def setUp(self):
         super().setUp()
         self.addCleanup(self.cleanup)
+        sys.meta_path.insert(0, self.meta_path_finder)
 
     def cleanup(self):
+        sys.meta_path.remove(self.meta_path_finder)
         # We use a flag and check it during the cleanup phase to fail the test
         # if needed. This is done because some module imports occur inside of a
         # try-except block that ignores all exceptions, so raising an exception