Bug#31398 problems with mysql-test-run

 - "mysqld --verbose --help" need to be started in a writable directory 


mysql-test/mysql-test-run.pl:
  Create a tmpdir where mysqld can peform it's lowercase or uppercase test

mysql-test/mysql-test-run.pl

@@ -1334,12 +1334,13 @@ sub datadir_list_setup () {
 
 sub collect_mysqld_features () {
   my $found_variable_list_start= 0;
+  my $tmpdir= tempdir(CLEANUP => 0); # Directory removed by this function
 
   #
   # Execute "mysqld --no-defaults --help --verbose" to get a
   # list of all features and settings
   #
-  my $list= `$exe_mysqld --no-defaults --verbose --help`;
+  my $list= `$exe_mysqld --no-defaults --datadir=$tmpdir --verbose --help`;
 
   foreach my $line (split('\n', $list))
   {
@@ -1394,7 +1395,7 @@ sub collect_mysqld_features () {
       }
     }
   }
-
+  rmtree($tmpdir);
   mtr_error("Could not find version of MySQL") unless $mysql_version_id;
   mtr_error("Could not find variabes list") unless $found_variable_list_start;