mtr: update heuristics for --parallel=auto

to work better for CPUs with more than 2000 bogomips.
old behavior is preserved if less than 2500 bogomips.

mysql-test/mysql-test-run.pl

@@ -441,11 +441,8 @@ sub main {
   if ( $opt_parallel eq "auto" ) {
     # Try to find a suitable value for number of workers
     my $sys_info= My::SysInfo->new();
+    $opt_parallel= $sys_info->num_cpus() + int($sys_info->min_bogomips()/500)-4;
 
-    $opt_parallel= $sys_info->num_cpus();
-    for my $limit (2000, 1500, 1000, 500){
-      $opt_parallel-- if ($sys_info->min_bogomips() < $limit);
-    }
     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);