* Consolidate testcontainer used by WebDriver test
Previously, we hard coded the version of the docker image for
provisioning Chrome browser and WebDriver server because the
version of the Chrome browser has to match the version of
the webdriver client, otherwise the screenshot test will fail.
Changing to use BrowserWebDriverContainer can delegate the match
to the library itself because it chooses the correct docker image
version based on the WebDriver version on our classpath.
* Increase maxColorDiff to 20
This is to supress the test flakness after switching to use
BrowserWebDriverContainer to provision browser and webdriver
service.
