Fully test the update check logic, by introducing several Qt tests.
Also, improve the `UpdaterThread.get_letest_info()` method, that gets
the latest version and changelog from GitHub, with several checks.
These checks are also tested in our newly added tests.
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.
