With this change, Karma tests can be run with an enforced Trusted Types policy.

This change consists of using safer APIs (appendChild and textContent instead of innerHTML), as well as creating a policy for client/karma.js which a test's Trusted Types CSP policy can then explicitly allow. This policy is used internally where karma does potentially dangerous operations like loading scripts.

More info about the proposed Trusted Types standard at https://github.com/WICG/trusted-types

…3348)

* feat(config): clientDisplayNone sets client elements display none.

When karma is used for compontent testing with screenshots, the client elements make screenshot comparison
more difficult.  config.client.clientDisplayNone will set the element style to display.none.

* feat(config): Document clientDisplayNone

This config runs through karmarunnerbot account and puts a comment on the PR. 
The older commit-blocking thing is still there.

Since of May 2019 by default start Ubuntu Xenial 16.04
https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment
and also since Ubuntu 16.04 you can start xvfb as services
https://docs.travis-ci.com/user/gui-and-headless-browsers/#using-xvfb-to-run-tests-that-require-a-gui

* fix(runner): remove explicit error on all tests failed

- there are failOnFailingTestSuite and failOnEmptyTestSuite, so in this there is no neccesity

Fixes #3367

* test(unit): Remove test

preprocessor plugins can execute 'done()' or return a Promise

The core-js was used for dom-serialize, used to support a single function
in stringify (serialize) for a single rare browser, IE8, in logs sent back to
the server.  By dropping this dependency we reduce the size of karma with
minimal impact on use cases.

Recently jasmine added support for reporting pending (incl due to fit()) tests.
This appears in results as skipped tests and this option allows us to fail tests
with skipped tests'