Improve tests to validate the correct contents of extracted files

[gfs]

Currently the tests primarily check that the correct number of files are returned from extraction commands. However, as the issue identified in #102 shows this can be insufficient in cases where the underlying library may not be threadsafe (for example). In such cases the tests can still succeed, finding the correct number of files, but their contents may be incomplete. We should develop additional tests that verify for each file type we support that the contents + sizes of each file are correct. Perhaps the easiest method would be with a hash of the contents of the extracted stream.