Changelog:
==========
* Actually test 7z support in the test_7z set of tests, not the lz4
functionality.
* In addition, correctly check for the 7z binary being available
(and not lz4) when testing 7z.
* Prevent a traceback when comparing a contentful .pyc file with an
empty one.
Signed-off-by: Wang Mingyu <wangmy@fujitsu.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>