Bug #47243 Allow MTR to read max auto setting for --parallel from env. variable

In stead of hard-coded 8, read from $MTR_MAX_PARALLEL if set

mysql-test/mysql-test-run.pl

@@ -322,7 +322,8 @@ sub main {
     for my $limit (2000, 1500, 1000, 500){
       $opt_parallel-- if ($sys_info->min_bogomips() < $limit);
     }
-    $opt_parallel= 8 if ($opt_parallel > 8);
+    my $max_par= $ENV{MTR_MAX_PARALLEL} || 8;
+    $opt_parallel= $max_par if ($opt_parallel > $max_par);
     $opt_parallel= $num_tests if ($opt_parallel > $num_tests);
     $opt_parallel= 1 if (IS_WINDOWS and $sys_info->isvm());
     $opt_parallel= 1 if ($opt_parallel < 1);