Re-add test runs against linux

Currently, we don't do any cross-plat compatibility. One of the workflows is set up to do that, but currently only runs against Windows. It is possible that tests that use this package could run on a Linux, so we should at a minimum get the tests set back up to use Ubuntu.

As part of this, it would be nice to turn the example Selenium tests back on. There are only a small number of them, so speed shouldn't be an issue.