Bug #43132 Pusbbuild 2 host sol10 sparc64 max is not running tests due to port unavailble

MTR gives up if wanted port not available
Try next range if set to 'auto'
Also, use next number for additional threads if explicitly set

mysql-test/lib/mtr_unique.pm

@@ -188,6 +188,8 @@ sub mtr_release_unique_id($) {
 
   flock SEM, LOCK_UN or warn "can't unlock $file.sem";
   close SEM;
+
+  delete $mtr_unique_ids{$$};
 }