br: reduce workload in unit test (pingcap#51493)

ref pingcap#48301

Author: Leavrth

tests/realtikvtest/brietest/backup_restore_test.go

@@ -79,7 +79,7 @@ func TestRestoreMultiTables(t *testing.T) {
 	tk.MustExec("use br")
 
 	tablesNameSet := make(map[string]struct{})
-	tableNum := 1000
+	tableNum := 100
 	for i := 0; i < tableNum; i += 1 {
 		tk.MustExec(fmt.Sprintf("create table table_%d (a int primary key, b json, c varchar(20))", i))
 		tk.MustExec(fmt.Sprintf("insert into table_%d values (1, '{\"a\": 1, \"b\": 2}', '123')", i))