test: fix unstable test TestScheduler (#7074)

close #7062

Signed-off-by: Cabinfever_B <cabinfeveroier@gmail.com>

Co-authored-by: ti-chi-bot[bot] <108142056+ti-chi-bot[bot]@users.noreply.github.com>

tests/pdctl/scheduler/scheduler_test.go

@@ -22,6 +22,7 @@ import (
 
 	"github.com/pingcap/kvproto/pkg/metapb"
 	"github.com/stretchr/testify/require"
+	"github.com/tikv/pd/pkg/core"
 	sc "github.com/tikv/pd/pkg/schedule/config"
 	"github.com/tikv/pd/pkg/utils/testutil"
 	"github.com/tikv/pd/pkg/versioninfo"
@@ -127,7 +128,8 @@ func TestScheduler(t *testing.T) {
 		pdctl.MustPutStore(re, leaderServer.GetServer(), store)
 	}
 
-	pdctl.MustPutRegion(re, cluster, 1, 1, []byte("a"), []byte("b"))
+	// note: because pdqsort is a unstable sort algorithm, set ApproximateSize for this region.
+	pdctl.MustPutRegion(re, cluster, 1, 1, []byte("a"), []byte("b"), core.SetApproximateSize(10))
 	time.Sleep(3 * time.Second)
 
 	// scheduler show command