Handle a few failing test cases

mysql-test/r/group_concat_max_len_func.result

@@ -76,4 +76,4 @@ id	rollno	group_concat(name)
 ## Dropping table t1 ##
 DROP table t1;
 ## Disconnecting both the connection ##
-SET @@group_concat_max_len = @save;
+SET @@global.group_concat_max_len = @save;

mysql-test/t/disabled.def

@@ -17,4 +17,5 @@ csv_alter_table      : Bug#33696 2008-01-21 pcrews no .result file - bug allows
 event_scheduler_basic: BUG#35997 server seems to crash.
 event_scheduler_func: BUG#35997 server seems to crash.
 thread_cache_size_func: BUG#35988 Due to not deterministic results
+user_limits     : Bug#23921 random failure of user_limits.test
 

mysql-test/t/group_concat_max_len_func.test

@@ -127,5 +127,5 @@ DISCONNECT test_con1;
 
 connection default;
 
-SET @@group_concat_max_len = @save;
+SET @@global.group_concat_max_len = @save;