pgsql: Fix "force_parallel_mode = regress" to work with ANALYZE + VERBO

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b925a00f4ef65db9359e1c60fbf0e56d05afb25a

Modified Files
--------------
src/backend/commands/explain.c | 27 +++++++++++++++++++++++----
src/include/commands/explain.h | 1 +
src/test/regress/expected/partition_prune.out | 10 +++++-----
src/test/regress/expected/subselect.out | 2 --
src/test/regress/sql/partition_prune.sql | 2 +-
src/test/regress/sql/subselect.sql | 2 --
6 files changed, 30 insertions(+), 14 deletions(-)