Make WCT tests less flaky

[x1ddos]

For instance, https://ci.cloudware.io/googlecodelabs/codelab-components/31:

02:06:11.228 INFO - Creating a new session for Capabilities [{platform=ANY, javascriptEnabled=true, browserName=chrome, chromeOptions={binary=/usr/bin/google-chrome, args=[start-maximized]}, version=48}]
Starting ChromeDriver 2.20.353124 (035346203162d32c80f1dce587c8154a1efa0c3b) on port 4348
Only local connections are allowed.
[info] Timeout request received, killing process

[x1ddos]

Won't fix. However, the drop-in replacement using just Custom Elements spec, without Polymer or other frameworks/libs in https://github.com/googlecodelabs/codelab-elements, provides a solid base for testing.