Run select_parallel test by itself

Remove the plpgsql wrapping that hides the context.  So now the test
will fail if the work doesn't actually happen in a parallel worker.  Run
the test in its own test group to ensure it won't run out of resources
for that.

src/test/regress/expected/select_parallel.out

@@ -111,14 +111,8 @@ explain (costs off)
          Index Cond: (unique1 = 1)
 (5 rows)
 
-do $$begin
-  -- Provoke error, possibly in worker.  If this error happens to occur in
-  -- the worker, there will be a CONTEXT line which must be hidden.
-  perform stringu1::int2 from tenk1 where unique1 = 1;
-  exception
-	when others then
-		raise 'SQLERRM: %', sqlerrm;
-end$$;
-ERROR:  SQLERRM: invalid input syntax for integer: "BAAAAA"
-CONTEXT:  PL/pgSQL function inline_code_block line 7 at RAISE
+-- provoke error in worker
+select stringu1::int2 from tenk1 where unique1 = 1;
+ERROR:  invalid input syntax for integer: "BAAAAA"
+CONTEXT:  parallel worker
 rollback;

src/test/regress/parallel_schedule

@@ -92,7 +92,10 @@ test: brin gin gist spgist privileges init_privs security_label collate matview
 test: alter_generic alter_operator misc psql async dbsize misc_functions
 
 # rules cannot run concurrently with any test that creates a view
-test: rules psql_crosstab select_parallel amutils
+test: rules psql_crosstab amutils
+
+# run by itself so it can run parallel workers
+test: select_parallel
 
 # ----------
 # Another group of parallel tests

src/test/regress/sql/select_parallel.sql

@@ -44,13 +44,7 @@ set force_parallel_mode=1;
 explain (costs off)
   select stringu1::int2 from tenk1 where unique1 = 1;
 
-do $$begin
-  -- Provoke error, possibly in worker.  If this error happens to occur in
-  -- the worker, there will be a CONTEXT line which must be hidden.
-  perform stringu1::int2 from tenk1 where unique1 = 1;
-  exception
-	when others then
-		raise 'SQLERRM: %', sqlerrm;
-end$$;
+-- provoke error in worker
+select stringu1::int2 from tenk1 where unique1 = 1;
 
 rollback;