[lldb/test] Add events listener helper class to lldbtest

This patch introduces a generic helper class that will listen for
event in a background thread and match it against a source broadcaster.

If the event received matches the source broadcaster, the event is
queued up in a list that the user can access later on.

The motivation behind this is to easily test new kinds of events
(i.e. Swift type-system progress events). However, this patch also
updates `TestProgressReporting.py` and `TestDiagnosticReporting.py`
to make use of this new helper class.

Differential Revision: https://reviews.llvm.org/D121977



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>