Setup improvements: download code and environment configuration

This abstracts some of the changes I made in the curly quotes plugin and drops them in the test harness so they can be shared across the plugins.

The setup tests script is virtually the same as the initial version except for a more explicit check for the presence of the Chromedriver.

If also made the SauceLabs reporting conditional on the authorisation being present in the environment as otherwise the test suite blows up with an error right at the end.