go/internal/xtesting: Add missing X

staticcheck says:

	xtesting.go:386:2: this value of err is never used (SA4006)

go/internal/xtesting/xtesting.go

@@ -383,7 +383,7 @@ func DrvTestWatch(t *testing.T, zurl string, zdrvOpen zodb.DriverOpener) {
 
 	ctx := context.Background()
 	watchq := make(chan zodb.Event)
-	zdrv, at0, err := zdrvOpen(ctx, u, &zodb.DriverOptions{ReadOnly: true, Watchq: watchq})
+	zdrv, at0, err := zdrvOpen(ctx, u, &zodb.DriverOptions{ReadOnly: true, Watchq: watchq}); X(err)
 	if at0 != at {
 		t.Fatalf("opened @ %s  ; want %s", at0, at)
 	}