use the right way to wait tiflash replica ready in least_greatest.test (

pingcap#4317)

close pingcap#4316

tests/fullstack-test/expr/least_greatest.test

@@ -25,7 +25,7 @@ mysql> create table test.t3 (id1 int, id2 int)
 mysql> alter table test.t3 set tiflash replica 1
 mysql> insert into test.t3 values (NULL, NULL), (NULL, NULL), (NULL, NULL)
 
-func> wait_table test t1, t2
+func> wait_table test t1 t2 t3
 
 # parse error
 mysql> set @@tidb_isolation_read_engines='tiflash'; set tidb_enforce_mpp=1; select least() from test.t1