Put back intra-grant-inplace.spec test coverage

Commit d31bbfb659 lost some test coverage, because the situation
being tested, a concurrent DROP, cannot happen anymore.  Put the test
coverage back with a bit of a trick, by deleting directly from the
catalog table.

Co-authored-by: Noah Misch <noah@leadboat.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://www.postgresql.org/message-id/flat/bf72b82c-124d-4efa-a484-bb928e9494e4@eisentraut.org

src/test/isolation/expected/intra-grant-inplace.out

@@ -226,7 +226,7 @@ step revoke4: <... completed>
 starting permutation: b1 drop1 b3 sfu3 revoke4 c1 r3
 step b1: BEGIN;
 step drop1: 
-	DROP TABLE intra_grant_inplace;
+	DELETE FROM pg_class WHERE relname = 'intra_grant_inplace';
 
 step b3: BEGIN ISOLATION LEVEL READ COMMITTED;
 step sfu3: 
@@ -248,6 +248,6 @@ relhasindex
 -----------
 (0 rows)
 
-s4: WARNING:  got: relation "intra_grant_inplace" does not exist
+s4: WARNING:  got: cache lookup failed for relation REDACTED
 step revoke4: <... completed>
 step r3: ROLLBACK;

src/test/isolation/specs/intra-grant-inplace.spec

@@ -20,7 +20,7 @@ step grant1	{
 	GRANT SELECT ON intra_grant_inplace TO PUBLIC;
 }
 step drop1	{
-	DROP TABLE intra_grant_inplace;
+	DELETE FROM pg_class WHERE relname = 'intra_grant_inplace';
 }
 step c1	{ COMMIT; }