Fix tests (#5050)

* Use sudo to fix chrome testing

There is a bug with the chrome sandbox in the Travis CI container
system, which is fixed by using `sudo: required`.

Related to travis-ci/travis-ci#8836

* reduce cognitive overhead of the test runner

* remove testing polyfilled Custom Elements with native ShadowDOM

* Use headless chrome and firefox

Use MacOS 10.13 for Safari 11

* disable testing CSS Custom Property Shim

* move sauce tests into script for travis_wait compat

.travis.yml

@@ -1,5 +1,5 @@
 language: node_js
-sudo: false
+sudo: required
 dist: trusty
 node_js: stable
 addons:
@@ -15,9 +15,8 @@ before_script:
 - gulp update-types
 - git diff --exit-code
 script:
-- xvfb-run wct -l chrome
-- xvfb-run wct -l firefox
-- if [ "${TRAVIS_PULL_REQUEST}" = "false" ]; then wct -s 'windows 10/microsoftedge@14' -s 'windows 10/microsoftedge@15' -s 'windows 8.1/internet explorer@11' -s 'os x 10.11/safari@9' -s 'macos 10.12/safari@10' -s 'macos 10.12/safari@11' -s 'Linux/chrome@41'; fi
+- wct -l chrome -l firefox
+- if [ "${TRAVIS_PULL_REQUEST}" = "false" ]; then travis_wait 30 ./util/travis-sauce-test.sh; fi
 env:
   global:
   - secure: bfF/o1ewpOxDNqTzWfvlwgRgGfP8OXhSQLLdEwZ6izO9tckMJuSNghk3qBXCEQJwTcUEyXP6EqfzIrRAvDXPa0H3OoinbrooDyV2wIDaVRK++WR2iZIqzqo3hGOdzm4tdrGJZe5av5Rk661Hls8aPfLbjdzcGuYXi8B4wZq2xMI=