run-tests: fix JUnit counts

In 19680f8, I forgot about my own TODO. At least, now I understand what
this line was about :P

run-tests.php

@@ -1764,7 +1764,7 @@ function run_worker(): void
                     "type" => "tests_finished",
                     "junit" => $junit->isEnabled() ? $junit : null,
                 ]);
-                //junit_init(); TODO is this needed?
+                $junit->clear();
                 break;
             default:
                 send_message($workerSock, [
@@ -3468,6 +3468,12 @@ public function isEnabled(): bool
         return $this->enabled;
     }
 
+    public function clear(): void
+    {
+        $this->rootSuite = self::EMPTY_SUITE + ['name' => 'php'];
+        $this->suites = [];
+    }
+
     public function saveXML(): void
     {
         if (!$this->enabled) {