MDEV-11719 main.subselect_no_exists_to_in failed in buildbot

main.log_slow might leave mysql.slow_log table non-empty,
and tests which later use it might fail. Make sure that the table
is properly truncated

mysql-test/r/log_slow.result

@@ -67,9 +67,9 @@ sleep(0.5)
 select count(*) FROM mysql.slow_log;
 count(*)
 1
-truncate mysql.slow_log;
 set @@long_query_time=default;
 set global slow_query_log= @org_slow_query_log;
 set @@log_slow_filter=default;
 set @@log_slow_verbosity=default;
 set global log_output= default;
+truncate mysql.slow_log;

mysql-test/t/log_slow.test

@@ -50,7 +50,6 @@ set global slow_query_log=1;
 set global log_output='TABLE';
 select sleep(0.5);
 select count(*) FROM mysql.slow_log;
-truncate mysql.slow_log;
 
 # Reset used variables
 set @@long_query_time=default;
@@ -58,3 +57,4 @@ set global slow_query_log= @org_slow_query_log;
 set @@log_slow_filter=default;
 set @@log_slow_verbosity=default;
 set global log_output= default;
+truncate mysql.slow_log;