`rake db:create` does not change permissions of root user.

Closes #8079.

I had to rework some of the tests because the mock allowed any arguments
for `connection.exeucte`. I think this is very dangerous as there could
anything be executed without the tests noticing it.
Merge pull request #9762 from senny/8079_do_not_grant_on_root_user

`rake db:create` does not change permissions of root user.