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.

0 files changed, 0 insertions, 0 deletions