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

This patch introduces 2 new lldb utility functions:
- lldbutil.start_listening_from: This can be called in the test setup to
  create a listener and set it up for a specific event mask and add it
  to the user-provided broadcaster's list.
- lldbutil.fetch_next_event: This will use fetch a single event from the
  provided istener and return it if it matches the provided broadcaster.

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 these new helper functions.

Differential Revision: https://reviews.llvm.org/D122193

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

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