Don't leave roles behind after core regression tests.
Commits
cf5eb37c5 and
e5b8a4c09 each created a new role that they
forgot to remove again. This breaks the use-case of running "make
installcheck" more than once, and it's also against project policy
because it'd be quite unfriendly behavior if one were running
"make installcheck" against a non-throwaway installation.