Drop global objects after completed test

Drop global objects after completed test

Project policy is to not leave global objects behind after a regress
test run.  This was found as a result of the development of a patch
to make pg_regress detect such leftovers automatically, which in the
end was withdrawn due to issues with parallel runs.

This was originally committed as 936e3fa3787a, but the issue also exists
in the 12~16 range.

Discussion: https://postgr.es/m/[email protected]
Backpatch-through: 12