Add minimal test for EXPLAIN ANALYZE of parallel query.

This displays the number of workers launched, thus the test is
dependant on configuration to some degree.  We'll see whether that
turns out ot be a problem.

Author: Rafia Sabih
Discussion: https://postgr.es/m/20170331185540.zmsue4ndvqtnayqw@alap3.anarazel.de

src/test/regress/expected/select_parallel.out

@@ -280,6 +280,16 @@ explain (costs off)
          Index Cond: (unique1 = 1)
 (5 rows)
 
+-- to increase the parallel query test coverage
+EXPLAIN (analyze, timing off, summary off, costs off) SELECT * FROM tenk1;
+                         QUERY PLAN                          
+-------------------------------------------------------------
+ Gather (actual rows=10000 loops=1)
+   Workers Planned: 4
+   Workers Launched: 4
+   ->  Parallel Seq Scan on tenk1 (actual rows=2000 loops=5)
+(4 rows)
+
 -- provoke error in worker
 select stringu1::int2 from tenk1 where unique1 = 1;
 ERROR:  invalid input syntax for integer: "BAAAAA"

src/test/regress/sql/select_parallel.sql

@@ -116,6 +116,9 @@ set force_parallel_mode=1;
 explain (costs off)
   select stringu1::int2 from tenk1 where unique1 = 1;
 
+-- to increase the parallel query test coverage
+EXPLAIN (analyze, timing off, summary off, costs off) SELECT * FROM tenk1;
+
 -- provoke error in worker
 select stringu1::int2 from tenk1 where unique1 = 1;