bugfix: mtr non-reliable failure detection

The $result variable was supposed to accumulate failures from
all mysqltest processes (that analyzed all error logs in parallel).
But it was mistakenly put in the loop, so it was reset for each process.

mysql-test/mysql-test-run.pl

@@ -4570,8 +4570,8 @@ sub check_warnings ($) {
 
   my $timeout= start_timer(check_timeout($tinfo));
 
+  my $result= 0;
   while (1){
-    my $result= 0;
     my $proc= My::SafeProcess->wait_any_timeout($timeout);
     mtr_report("Got $proc");