MDEV-32518 Test failure: ./mtr --no-reorder main.log_slow_debug main.subselect

There where two errors left from the previous fix.

- subselect.test assumes that mysql.slow_log is empty. This was not
  enforced.
- subselect.test dropped a file that does not exists (for safety).
  This was fixed by ensuring we don't get a warning if the file does
  not exist.

mysql-test/main/subselect_no_exists_to_in.result

@@ -7163,6 +7163,7 @@ drop table t1;
 #
 # MDEV-7565: Server crash with Signal 6 (part 2)
 #
+truncate table mysql.slow_log;
 Select 
 (Select Sum(`TestCase`.Revenue) From mysql.slow_log E           
 Where TestCase.TemplateID not in (Select 1 from mysql.slow_log where 2=2)