VcsIntegrationEnablerTest implemented for mock test vcs

*GitIntegrationEnablerTest changed to common VcsIntegrationEnablerTest;
*test notification methods moved to VcsTestUtil;
*cosmetics
Cleanup (pointless exceptions; typos; formatting)
IDEA-54387 (type annotations in introduce parameter)
Fixed Nullable/NotNull annotations.
Merge remote-tracking branch 'origin/master'