ouch! the ?? operator is right-associative! major whoopsie. that's wh…

…at you get for making quick edits on github without running the tests. what was I thinking.

src/TestDriver.php

@@ -381,7 +381,7 @@ public function trace(): string
         for ($i=count($traces)-1; $i>=0; $i--) {
             $trace = $traces[$i];
 
-            if ($trace['file'] ?? null === $current_file) {
+            if (($trace['file'] ?? null) === $current_file) {
                 $selected_trace = $trace;
             }
         }