fix(testWithSpectron): only stop spectron app if running, fixes #543 (#544)

Author: tomas-pecserke

lib/testWithSpectron.js

@@ -68,7 +68,7 @@ module.exports = (options = {}) =>
             child.stdin.write('close')
             child.kill()
             // Close spectron
-            if (app) return app.stop()
+            if (app && app.isRunning()) return app.stop()
           }
         })
       }