Add support for Grails random HTTP/HTTPS port.

Listen to the new test event that occurs after the test phase is
prepared and get the actual local server port if necessary.

Supports the changes made in https://github.com/grails/grails-core/commit/e4c63c60c907766d68205e5156d15d9e3318631b and https://github.com/grails-plugins/grails-tomcat-plugin/commit/19369183876e9aa590d3bc48f6b4d6f5c51c1899