Adjust regression test cases added by commit
ab7271677.
I suppose it is a copy-and-paste error that this test doesn't actually
test the "Parallel Append with both partial and non-partial subplans"
case (EXPLAIN alone surely doesn't qualify as a test of executor
behavior). Fix that.
Also, add cosmetic aliases to make it possible to tell apart these
otherwise-identical test cases in log_statement output.