In #6814 we started using the Python test runner for upgrade tests in
run_test.py, instead of the Perl based one. This had a problem though,
not all tests in minimal_schedule can be run with the Python runner.
This adds a separate minimal schedule for the pg_upgrade tests which
doesn't include the tests that break with the Python runner.
This PR also fixes various other issues that came up while testing
the upgrade tests.
Jelte Fennema