pgsql: check_exclusion_constraint didn't actually work correctly for

Log Message:
-----------
check_exclusion_constraint didn't actually work correctly for index
expressions: FormIndexDatum requires the estate's scantuple to already point
at the tuple the values are supposedly being extracted from.  Adjust test
case so that this type of confusion will be exposed.
Per report from hubert depesz lubaczewski.

Modified Files:
--------------
    pgsql/src/backend/executor:
        execUtils.c (r1.168 -> r1.169)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execUtils.c?r1=1.168&r2=1.169)
    pgsql/src/test/regress/input:
        constraints.source (r1.14 -> r1.15)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/input/constraints.source?r1=1.14&r2=1.15)
    pgsql/src/test/regress/output:
        constraints.source (r1.47 -> r1.48)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/output/constraints.source?r1=1.47&r2=1.48)