fix(test): unflake waitForSelector when browser closes test (#767)

The waitForSelector call might not have made it into the browser by the time we disconnect, resulting in a websocket error instead of a protocol error.

test/launcher.spec.js

@@ -172,6 +172,10 @@ module.exports.describe = function({testRunner, expect, defaultBrowserOptions, p
  const remote = await playwright.connect(browserApp.connectOptions());
  const page = await remote.defaultContext().newPage();
  const watchdog = page.waitForSelector('div', { timeout: 60000 }).catch(e => e);
+
+ // Make sure the previous waitForSelector has time to make it to the browser before we disconnect.
+ await page.waitForSelector('body');
+
  await remote.disconnect();
  const error = await watchdog;
  expect(error.message).toContain('Protocol error');