Actually test 7z support in the test_7z set of tests, not the lz4...

Actually test 7z support in the test_7z set of tests, not the lz4 functionality. (Closes: #359)

We also needed to rename the 7z.zip module to sevenz.py so it was actually
importable in regular Python code. Thanks to Vagrant Cascadian for spotting
this and for pointing it out so cordially. :)