pgsql: Fix test case to do what it intends to

Fix test case to do what it intends to

This test case intended to fail because setting a column as generated to
the partitioned table while leaving the partition alone is not allowed;
but instead failed because of a discrepancy of not-null constraint.  Fix
this by adding the not-null constraint first, then set the column as
generated in a separate ALTER TABLE command, which gets the expected
error.  Also, because the next test also wants to set the column as
not-null, add a BEGIN/ROLLBACK block so that the added not-null is
removed.

Oversight in 699586315704.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e89f4c66182e409b6643b1e8426371011dc25217

Modified Files
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src/test/regress/expected/identity.out | 6 ++++--
src/test/regress/sql/identity.sql      | 4 +++-
2 files changed, 7 insertions(+), 3 deletions(-)