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.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)

mysql-test/main/subselect.test

@@ -1171,8 +1171,10 @@ create table t1 (a int, b decimal(13, 3));
 insert into t1 values (1, 0.123);
 let $outfile_abs= $MYSQLTEST_VARDIR/tmp/subselect.out.file.1;
 let $outfile_rel= ../../tmp/subselect.out.file.1;
+--disable_warnings
 --error 0,1
 --remove_file $outfile_abs
+--enable_warnings
 eval select a, (select max(b) from t1) into outfile "$outfile_rel" from t1;
 delete from t1;
 eval load data infile "$outfile_rel" into table t1;
@@ -5990,6 +5992,8 @@ drop table t1;
 --echo #
 --echo # MDEV-7565: Server crash with Signal 6 (part 2)
 --echo #
+
+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)

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)

mysql-test/main/subselect_no_mat.result

@@ -7156,6 +7156,7 @@ 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)

mysql-test/main/subselect_no_opts.result

@@ -7154,6 +7154,7 @@ 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)

mysql-test/main/subselect_no_scache.result

@@ -7169,6 +7169,7 @@ 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)

mysql-test/main/subselect_no_semijoin.result

@@ -7154,6 +7154,7 @@ 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)