test RAM driver cant delete

driver_test.go

@@ -76,6 +76,12 @@ func testDriver(t *testing.T, d Driver) {
 		t.Error("cannot set from full:", err)
 	}
 
+	if err := d.ForEach(ctx, "m", func(string, []byte) error {
+		return errors.New("should have no hits")
+	}); err != nil {
+		t.Error("failed to forEach empty:", err)
+	}
+
 	if b, err := d.Get(ctx, "m", "id"); err != nil {
 		t.Error("cannot get from deleted:", err)
 	} else if b != nil {