[lldb/test] Re-enable TestEvents.py on Darwin and fix crashes

This patch re-enables TestEvents.py on Darwin and fixes some crashes
that were happening due to an undefined method.

I ran it 100 times locally with the following command and it passed
every the time:

```
for i in {1..100}; do print $i/100; ./bin/lldb-dotest -p TestEvents.py 2>&1 | rg PASSED; if [ "$?" -eq "1" ]; then break; fi; done
```

Let's see if it still fails non-deterministically on the bots and
eventually also re-enable it on linux.

rdar://37037235

Differential Revision: https://reviews.llvm.org/D120607



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