Fix ALTER TABLE error message

This bogus error message was introduced in 2013 by commit f177cbfe67,
because of misunderstanding the processCASbits() API; at the time, no
test cases were added that would be affected by this change.  Only in
ca87c415e2 was one added (along with a couple of typos), with an XXX
note that the error message was bogus.  Fix the whole, add some test
cases.

Backpatch all the way back.

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/202503041822.aobpqke3igvb@alvherre.pgsql

src/backend/parser/gram.y

@@ -2318,7 +2318,7 @@ alter_table_cmd:
 					n->def = (Node *) c;
 					c->contype = CONSTR_FOREIGN; /* others not supported, yet */
 					c->conname = $3;
-					processCASbits($4, @4, "ALTER CONSTRAINT statement",
+					processCASbits($4, @4, "FOREIGN KEY",
 									&c->deferrable,
 									&c->initdeferred,
 									NULL, NULL, yyscanner);