Now that sample_doc was renamed to sample_pdf it could cause some
confusion the fact that that the TestBase class had an attribute called
sample_doc which referenced the sample PDF.
By removing this attribute and passing the fixture instead we are
following a more pytest-native approach of passing arguments explicitly.
Add a Pytest fixture that returns an UpdaterThread instance which has
its own unique settings directory. Note that the UpdaterThread instance
needs to be slightly nerfed, so that it doesn't rely on Qt functionality
or any isolation providers.
