Refactored tests.

[afourney]

This PR substantially increases test coverage by refactoring tests such that each file is tested through each entry point (local_file, streaming, and by url), on both the CLI and the Python module.

This is critical because different entry points convey different information about the file being converted. For example:

• if a local file is converted, the filename and extension is known, but not the mime type or character encoding
• if a url is converted, the filename may not be known, but the mimetype and charset are often in the response header
• if a stream (e.g, stdin) is being converted, nothing might be known from the stream

Prior test coverage failed to check all these combinations, leading to several prior bugs.

[afourney]

Linking the special casing of 'test_mskanji.csv' in the stream tests to: google/magika#983

Confirmed via MD5 sum, and minimal test reproduction (see google/magika#983) that the correct bytes are being sent.