pgsql: ATPrepAddPrimaryKey: ignore non-PK constraints

ATPrepAddPrimaryKey: ignore non-PK constraints

Because of lack of test coverage, this function added by b0e96f311985
wasn't ignoring constraint types other than primary keys, which it
should have.  Add some lines to a test for it.

Reported-by: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/CAMbWs48bc-k_-1fh0dZpAhp_LiR5MfEX9haystmoBboR_4czCQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e09d763e25dccc40695bc824ddda9abea791d66f

Modified Files
--------------
src/backend/commands/tablecmds.c      | 12 +++++++++---
src/test/regress/expected/inherit.out | 25 ++++++++++++++++++++++++-
src/test/regress/sql/inherit.sql      | 15 ++++++++++++++-
3 files changed, 47 insertions(+), 5 deletions(-)