test_runner: Write database ids to file for reference in clean up.

The ids that will be used for each particular run of the test suite are
written to a unique file. Each file will then be used as a time
reference of when the suite was ran.

This change sets up the ability for a complete clean up of potentially
leaked database templates.

Tweaked by tabbott to remove these files after successful database
cleanup.

scripts/lib/zulip_tools.py

@@ -145,6 +145,7 @@ if __name__ == '__main__':
     if cmd == 'make_deploy_path':
         print(make_deploy_path())
 
+TEMPLATE_DATABASE_DIR = "test-backend/databases"
 def get_dev_uuid_var_path(create_if_missing=False):
     # type: (bool) -> str
     zulip_path = get_deploy_root()