Improve code coverage of select_parallel test.

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

Commit: 4c7af96365d6a50c4f807a97dd37ac3e9c7459cf
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2017-08-31T17:15:54Z
Releases: 10.0
Improve code coverage of select_parallel test.

Make sure that rescans of parallel indexscans are tested.
Per code coverage report.

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