MDEV-27448 MTR returns success (zero) upon invalid option

I change from `exit;` to `exit(1);` on a function `usage()`.
When we try to run mtr with a wrong option, a function `usage()` is called with the wrong option as its argument. In this case, because the function call `exit` in a first if statement, we get exit status 0.

mysql-test/mysql-test-run.pl

@@ -5726,7 +5726,7 @@ sub usage ($) {
   {
     print STDERR "$message\n";
     print STDERR "For full list of options, use $0 --help\n";
-    exit;      
+    exit(1);
   }
 
   local $"= ','; # for @DEFAULT_SUITES below