Adds tests for macOS and Windows with the dummy converter. Tests won't
actually perform the conversion. But it should be enough for us to test
the remainder of the codebase.
Fixes#229
Checking if files were writeable created files in the process. In the
case where someone adds a list of N files to dangerzone but exits before
converting, they would be left with N 0-byte files for the -safe
version. Now they don't.
Fixes#214
Let the Document class suggest the default filename for the safe PDF,
based on the provided input filename, appended with the extension
`-safe.pdf`.
Previously, this logic was copy-pasted throughout the code, which made
it difficult to maintain.
Run Mypy static checks against our tests. This brings them inline with
the rest of the codebase, and we have an extra level of certainty that
the tests (and unit tests in particular) will not significantly diverge
from the code they are testing.
