Adjust regression test cases added by commit ab7271677.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 979a36c3894db0a4b0d6b4b20fc861a0bbe3271c
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2017-12-06T03:40:43Z
Releases: 11.0
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.

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