The reason for test failures was that there weren't records for
accounts root@127.0.0.1 and root@::1 in the table mysql.user.
These records were removed from installation script within implementation
of WL#7158. In result it led to access denied error on any attempt to connect
to a server on behalf of user listed above.

To resolve this bug records for users root@127.0.0.1 and root@::1 are added
explicitly into the table mysql.user in every test file that relies on the
presence of records for accounts root@127.0.0.1 and root@::1.

Also this patch makes refactoring of test cases to move adding/removing of
users root@127.0.0.1, root@::1 into a separate files that are called
from other test cases.