Automate replacement of content.src in config.xml

https://github.com/apache/cordova-plugin-test-framework#running-plugin-tests

> Change the start page in config.xml with or add a link to cdvtests/index.html from within your app.

This can probably be automated via `plugin.xml`, edit-config or config-file.

This would be a BREAKING CHANGE

Important: When this is added, https://github.com/apache/cordova-mobile-spec has to be fixed to itself change it back to `index.html` so that the overview page is loaded instead of what this plugin will want to load.