test_runner: make it compatible with fake timers

Signed-off-by: Matteo Collina <hello@matteocollina.com>

Author: mcollina

lib/internal/test_runner/test.js

@@ -149,7 +149,7 @@ function stopTest(timeout, signal) {
 
     disposeFunction = () => {
       abortListener[SymbolDispose]();
-      timer[SymbolDispose]();
+      clearTimeout(timer);
     };
   }