Add Parallel Bitmap Heap Scan case

[avamingli]

Issue #65 says we need to support Parallel Bitmap Heap Scan.
I don't have the impression that we disable it at some time.
Have a check and yeah, we have it, but lack of a case.
Add a case to show that.

explain(costs off) select c2 from t1;
                 QUERY PLAN
--------------------------------------------
 Gather Motion 6:1  (slice1; segments: 6)
   ->  Parallel Bitmap Heap Scan on t1
         ->  Bitmap Index Scan on t1_c2_idx

By adding the cases, found that the underling Bitmap Index Scan Locus is NULL.
It's not so right and may cause confusion even it doesn't have an impact on Plan Execution.

explain(locus, costs off) select c2 from t1;
                 QUERY PLAN                 
--------------------------------------------
 Gather Motion 6:1  (slice1; segments: 6)
   Locus: Entry
   ->  Parallel Bitmap Heap Scan on t1
         Locus: HashedWorkers
         Parallel Workers: 2
         ->  Bitmap Index Scan on t1_c2_idx
               Locus: NULL.    <-------------------
 Optimizer: Postgres query optimizer
(8 rows)

I have new an issue #86 to track that.

Authored-by: Zhang Mingli avamingli@gmail.com

closes: #65

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[wfnuser]

LGTM.

[sandiandian]

There are times when test may fail due to instability and you can click retry without having to re-trigger the whole ci.