backfill: skip TestVectorIndexMergingDuringBackfillWithPrefix under race

We've seen this recently added test time out twice under race with no
clear signs of anything going wrong. The test seems just rather intense
to be run under race, so we'll skip it in that config.

Release note: None

Author: yuzefovich

pkg/sql/backfill/BUILD.bazel

@@ -87,6 +87,7 @@ go_test(
         "//pkg/sql/sem/catid",
         "//pkg/sql/sem/eval",
         "//pkg/testutils/serverutils",
+        "//pkg/testutils/skip",
         "//pkg/testutils/sqlutils",
         "//pkg/testutils/testcluster",
         "//pkg/util/leaktest",

pkg/sql/backfill/backfill_test.go

@@ -14,6 +14,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/kv"
 	"github.com/cockroachdb/cockroach/pkg/sql/execinfra"
 	"github.com/cockroachdb/cockroach/pkg/testutils/serverutils"
+	"github.com/cockroachdb/cockroach/pkg/testutils/skip"
 	"github.com/cockroachdb/cockroach/pkg/testutils/sqlutils"
 	"github.com/cockroachdb/cockroach/pkg/util/leaktest"
 	"github.com/cockroachdb/cockroach/pkg/util/log"
@@ -339,6 +340,8 @@ func TestVectorIndexMergingDuringBackfillWithPrefix(t *testing.T) {
 	defer leaktest.AfterTest(t)()
 	defer log.Scope(t).Close(t)
 
+	skip.UnderRace(t, "too slow")
+
 	// Channel to block the backfill process
 	blockBackfill := make(chan struct{})
 	backfillBlocked := make(chan struct{})