Implement source position display on test failure

Test.pm6

@@ -1,9 +1,20 @@
-my module Test;
+module Test; # XXX our due to the STD pad bug
+
+constant $?TRANSPARENT = 1;
+
+sub blame() {
+    my $frame = caller;
+    while $frame.hints('$?TRANSPARENT') {
+        $frame = $frame.caller;
+    }
+    $frame.file ~ (" line " ~ $frame.line);
+}
 
 my $testnum = 1;
 sub ok($bool, $tag) is export {
     my $not = (if $bool { "" } else { "not " });
     say ($not ~ ("ok " ~ ($testnum++ ~ (" - " ~ $tag))));
+    if !$bool { say ("# " ~ blame()); }
 }
 
 sub plan($num) is export {