Add test coverage to stacks actions

[fg-j]

The get-usns Github action used for polling the Ubuntu USN RSS feed is one of the most important actions in the Paketo ecosystem -- it's the main mechanism that triggers automatic stack updates that keep our stacks secure for users. Currently, it's untested. We should add tests. Many of these may mock out an RSS feed server.

In addition, I think it'd be useful to have a test that will fail if the RSS feed or Ubuntu USN/CVE web pages change their format in a way that makes our data scraping invalid. For instance, asserting that scraping a real USN page for a certain regex returns a reasonable number of matches, a valid URL, etc.

The diff-package-receipts and release-notes actions are also written in Golang and have some degree of complexity. These should be tested as well.