[vttestserver] Fix errcheck violations in new tests

Signed-off-by: Andrew Mason <amason@slack-corp.com>

go/cmd/vttestserver/vttestserver_test.go

@@ -135,7 +135,7 @@ func TestForeignKeysAndDDLModes(t *testing.T) {
 	assert.NoError(t, err)
 	defer cluster.TearDown()
 
-	execOnCluster(cluster, "test_keyspace", func(conn *mysql.Conn) error {
+	err = execOnCluster(cluster, "test_keyspace", func(conn *mysql.Conn) error {
 		_, err := conn.ExecuteFetch(`CREATE TABLE test_table_2 (
 			id BIGINT,
 			test_table_id BIGINT,
@@ -154,13 +154,14 @@ func TestForeignKeysAndDDLModes(t *testing.T) {
 		assert.NoError(t, err)
 		return nil
 	})
+	assert.NoError(t, err)
 
 	cluster.TearDown()
 	cluster, err = startCluster("-foreign_key_mode=disallow", "-enable_online_ddl=false", "-enable_direct_ddl=false")
 	assert.NoError(t, err)
 	defer cluster.TearDown()
 
-	execOnCluster(cluster, "test_keyspace", func(conn *mysql.Conn) error {
+	err = execOnCluster(cluster, "test_keyspace", func(conn *mysql.Conn) error {
 		_, err := conn.ExecuteFetch(`CREATE TABLE test_table_2 (
 			id BIGINT,
 			test_table_id BIGINT,
@@ -177,6 +178,7 @@ func TestForeignKeysAndDDLModes(t *testing.T) {
 		assert.Error(t, err)
 		return nil
 	})
+	assert.NoError(t, err)
 }
 
 func TestCanVtGateExecute(t *testing.T) {