[COMMITTERS] pgsql: Improve code coverage of select_parallel test.

Improve code coverage of select_parallel test.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4b1dd62a257a469f92fef4f4cce37beab6c0b98b

Modified Files
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src/test/regress/expected/select_parallel.out | 55 +++++++++++++++++++++++++++
src/test/regress/sql/select_parallel.sql      | 20 ++++++++++
2 files changed, 75 insertions(+)