Remove any old pidfile before starting mysqld to make sure that

'mysqld_wait_started' don't return prematurely because of an old
pidfile

mysql-test/mysql-test-run.pl

@@ -3825,6 +3825,9 @@ sub mysqld_start ($$$) {
     $wait_for_pid_file= 0;
   }
 
+  # Remove the pidfile
+  unlink($mysqld->{'path_pid'});
+
   if ( defined $exe )
   {
     $pid= mtr_spawn($exe, $args, "",