./mtr --skip-not-found

New mtr option --skip-not-found makes it to show not found tests
as skipped

main.a                                   [ skipped ]  not found

(but only if the test was specified with the suite name)
and not error out early with

mysql-test-run: *** ERROR: Could not find 'a' in 'main' suite

This is useful in buildbot, on builders that generate the list
of tests dynamically.

mysql-test/lib/mtr_cases.pm

@@ -154,7 +154,17 @@ sub collect_test_cases ($$$$) {
         {
 	  push (@$cases, @this_case);
 	}
-	else
+	elsif ($::opt_skip_not_found)
+        {
+          push @$cases, My::Test->new
+            (
+             name          => "$sname.$tname",
+             shortname     => $tname,
+             skip          => 1,
+             comment       => 'not found',
+            );
+        }
+        else
 	{
 	  mtr_error("Could not find '$tname' in '$sname' suite");
         }