Bug#38120: main.partition fails sporadically

sporadic failures due to full disk.

Fix by truncating general_log before altering it.
(if running the full main-test, it can be big).

mysql-test/r/partition.result:
  Bug#38120: main.partition fails sporadically
  
  updated result file
mysql-test/t/partition.test:
  Bug#38120: main.partition fails sporadically
  
  Truncating general_log before altering it.
  (if running the full main-test, it can be big
  when altering it).

mysql-test/r/partition.result

@@ -1448,13 +1448,15 @@ create trigger t_ad after delete on t for each row insert into t values (old.s1)
 insert into t values (1);
 drop table t;
 USE mysql;
+TRUNCATE TABLE general_log;
+SET @old_general_log_state = @@global.general_log;
 SET GLOBAL general_log = 0;
 ALTER TABLE general_log ENGINE = MyISAM;
 ALTER TABLE general_log PARTITION BY RANGE (TO_DAYS(event_time))
 (PARTITION p0 VALUES LESS THAN (733144), PARTITION p1 VALUES LESS THAN (3000000));
 ERROR HY000: Incorrect usage of PARTITION and log table
 ALTER TABLE general_log ENGINE = CSV;
-SET GLOBAL general_log = default;
+SET GLOBAL general_log = @old_general_log_state;
 use test;
 create table t2 (b int);
 create table t1 (b int)

mysql-test/t/partition.test

@@ -1589,13 +1589,15 @@ drop table t;
 #
 
 USE mysql;
+TRUNCATE TABLE general_log;
+SET @old_general_log_state = @@global.general_log;
 SET GLOBAL general_log = 0;
 ALTER TABLE general_log ENGINE = MyISAM;
 --error ER_WRONG_USAGE
 ALTER TABLE general_log PARTITION BY RANGE (TO_DAYS(event_time))
   (PARTITION p0 VALUES LESS THAN (733144), PARTITION p1 VALUES LESS THAN (3000000));
 ALTER TABLE general_log ENGINE = CSV;
-SET GLOBAL general_log = default;
+SET GLOBAL general_log = @old_general_log_state;
 use test;
 
 #