tests: Improve and add vCard tests

Add a few more vCard tests to make sure we're not regressing on our vCard import feature.

One of the design desicions at the time was to make the Contacts.Io.Parser take a GLib.InputStream so we can make it agnostic of what kind of input was given. We can now use this to make the vCard tests a bit simpler in general by using a constant string inside the test file rather than having to deal with environment variables to pass on a path.