add `list_failed_test_files` command, bound to `l`

https://github.com/sunaku/tork/pull/60
engine: emit test file counts in :idle event data
engine: emit actual test files in :idle event data
engine: only list most recent test files in :idle
Merge branch 'other'
Merge branch 'patch'
Merge branch 'minor'
Version 19.10.0 (2014-08-06)