Add a test for adding and removing Listeners from a BroadcasterManager.

Reid found a bug in removing Listeners from a BroadcasterManager:

https://reviews.llvm.org/D74010

The bug didn't affect the case where there was only one Listener
signed up for a BroadcasterManager, which was all the extant test
case tests.  The driver also only uses one listener (the debugger)
for everything, so neither the test nor anything you do with lldb
command line would have triggered the bug.

This adds a couple more tests using more listeners, and adding and
removing them in a different way, which triggers a separate code path.