Avoid "WebSocket is closed before the connection is established." error in the end of a test-run

[SimonSimCity]

I found out that the sometimes occurring error WebSocket is closed before the connection is established. can be avoided by not explicitly closing the page but the browser directly.

The error message normally looks like this:

--------------------------------
----- RUNNING CLIENT TESTS -----
--------------------------------
Kadira: completed instrumenting the app
HeadlessChrome/68.0.3419.0
=> Started your app.

=> App running at: http://localhost:3100/
Kadira: authentication failed - check your appId & appSecret
1..5
ok 1 sandbox executes code
ok 2 sandbox can run with an object as `this`
ok 3 sandbox uses an empty string as `this` if not defined
ok 4 sandbox can run with undefined arguments
ok 5 sandbox returns a function with no return if functionBody is empty
# tests 5
# pass 5
# fail 0
(node:58667) UnhandledPromiseRejectionWarning: Error: Protocol error (Runtime.evaluate): Session closed. Most likely the page has been closed.
    at CDPSession.send (/meteor-project/node_modules/puppeteer/lib/Connection.js:187:29)
    at ExecutionContext.evaluateHandle (/meteor-project/node_modules/puppeteer/lib/ExecutionContext.js:67:77)
    at ExecutionContext.evaluate (/meteor-project/node_modules/puppeteer/lib/ExecutionContext.js:46:31)
    at Frame.evaluate (/meteor-project/node_modules/puppeteer/lib/FrameManager.js:326:20)
    at <anonymous>
    at process._tickDomainCallback (internal/process/next_tick.js:228:7)
 => awaited here:
    at Function.Promise.await (/Users/test/.meteor/packages/promise/.0.10.2.4xp0lj.vhtlc++os+web.browser+web.cordova/npm/node_modules/meteor-promise/promise_server.js:56:12)
    at Promise.asyncApply (packages/meteortesting:browser-tests/browser/puppeteer.js:48:13)
    at /Users/test/.meteor/packages/promise/.0.10.2.4xp0lj.vhtlc++os+web.browser+web.cordova/npm/node_modules/meteor-promise/fiber_pool.js:43:40
(node:58667) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 2)
(node:58667) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
=> Exited with code: 0

The message the system tries to report while being killed off is:

ConsoleMessage {
  _type: 'warning',
  _text: 'WebSocket connection to \'ws://localhost:3100/sockjs/473/wltn30p9/websocket\' failed: WebSocket is closed before the connection is established.',
  _args: [] }