pgsql: Add more subscription DDL tests

Add more subscription DDL tests

Add more tests for various variants of subscription DDL commands, based
on code coverage report.  Fix a small bug discovered by that.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/e3920ac823353f200cfecde74076d3d40f79e2b1

Modified Files
--------------
src/backend/commands/subscriptioncmds.c    |  2 +-
src/test/regress/expected/subscription.out | 29 +++++++++++++++++++++++++++++
src/test/regress/sql/subscription.sql      | 27 +++++++++++++++++++++++++++
3 files changed, 57 insertions(+), 1 deletion(-)