[lldb/test] Update lldbutil.fetch_next_event to match broadcaster class

This patch updates the `lldbutil.fetch_next_event` helper function to
either match a specific broadcaster or match a whole broadcaster class.

This is very handy when testing process events for interactive scripted
process debugging.

This also fixes a bug in the failing case, where `SBEvent.GetDescription`
expects a `SBStream` argument. We never took that code path in the
original implementation so we didn't hit that bug.

Differential Revision: https://reviews.llvm.org/D149175



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