[Patch] Fix pg_upgrade/t/007_multixact_conversion.pl

Hello,

This about the test pg_upgrade/t/007_multixact_conversion.pl. It was added several months ago. The test creates table mxofftest :
CREATE TABLE mxofftest (id INT PRIMARY KEY, n_updated INT)
Next there is wrong usage of this table:
"SELECT ctid, xmin, xmax, * FROM mxofftest");
It should be
"SELECT id, n_updated, * FROM mxofftest");

The fix is very simple, the patch attached.

Best regards,
Alexander Potapov