*: make TestStores stable (#8027)

close #7954

Signed-off-by: Ryan Leung <rleungx@gmail.com>

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

tests/integrations/mcs/scheduling/api_test.go

@@ -650,6 +650,8 @@ func (suite *apiTestSuite) checkStores(cluster *tests.TestCluster) {
 	for _, store := range stores {
 		tests.MustPutStore(re, cluster, store)
 	}
+	// prevent the offline store from changing to tombstone
+	tests.MustPutRegion(re, cluster, 3, 6, []byte("a"), []byte("b"))
 	// Test /stores
 	apiServerAddr := cluster.GetLeaderServer().GetAddr()
 	urlPrefix := fmt.Sprintf("%s/pd/api/v1/stores", apiServerAddr)