Summary: The test shouldn't run if the dependency (lz4) cannot be imported.
Test Plan:
Run the test on a machine that does not have Python lz4 module installed and
make sure the test gets skipped.
Reviewers: durham, #mercurial
Reviewed By: durham
Subscribers: durham
Differential Revision: https://phabricator.intern.facebook.com/D6678494
Signature: 6678494:1515454247:245401173d9e1ef16ab865c210b1f5412039c1e1