Fix "force_parallel_mode = regress" to work with ANALYZE + VERBOSE.

force_parallel_mode = regress is supposed to force use of a Gather
node without having any impact on EXPLAIN output.  But it failed to
accomplish that if both ANALYZE and VERBOSE are given, because that
enables per-worker output data that you wouldn't see if the Gather
hadn't been inserted.  Improve the logic so that we suppress the
per-worker data too.

This allows putting the new test case added by commit 5935917ce
back into the originally intended form (cf. 776a2c887, 22864f6e0).
We can also get rid of a kluge in subselect.sql, which previously
had to clean up after force_parallel_mode's failure to do what it
said on the tin.

Discussion: https://postgr.es/m/18445.1576177309@sss.pgh.pa.us

src/include/commands/explain.h

@@ -46,6 +46,7 @@ typedef struct ExplainState
 	List	   *rtable_names;	/* alias names for RTEs */
 	List	   *deparse_cxt;	/* context list for deparsing expressions */
 	Bitmapset  *printed_subplans;	/* ids of SubPlans we've printed */
+	bool		hide_workers;	/* set if we find an invisible Gather */
 } ExplainState;
 
 /* Hook for plugins to get control in ExplainOneQuery() */