Refactor: cts-lab: Drop SearchObj.cache member

It gets set to empty at each harvest_async() call. Then during
async_complete, it gets built up line-by-line, sent to
self._delegate.async_complete() (which only ever belongs to LogWatcher),
and never used again.

It's clearer to drop the member and use a local variable called
messages.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>