Event subscription in server_init_hook can crash

CI tests show that this feature is unreliable. Occasionally, the test times out, but more concerning it can segfault (Linux, and macOS).

We need to get a call stack show C++ code as well, but here is the Python call stack.

tests/test_server.py::test_server_init_hook_subscribe_event_multiple_devices Fatal Python error: Segmentation fault
Current thread 0x00007fc4439ca700 (most recent call first):
  File "/builds/[MASKED]/pytango/venv/lib/python3.9/site-packages/tango/device_proxy.py", line 1403 in __DeviceProxy__subscribe_event_attrib
  File "/builds/[MASKED]/pytango/venv/lib/python3.9/site-packages/tango/device_proxy.py", line 1330 in __DeviceProxy__subscribe_event
  File "/builds/[MASKED]/pytango/venv/lib/python3.9/site-packages/tango/green.py", line 112 in run
  File "/builds/[MASKED]/pytango/venv/lib/python3.9/site-packages/tango/green.py", line 204 in greener
  File "/builds/[MASKED]/pytango/tests/test_server.py", line 3581 in server_init_hook
  File "/builds/[MASKED]/pytango/venv/lib/python3.9/site-packages/tango/green.py", line 101 in execute
  File "/builds/[MASKED]/pytango/venv/lib/python3.9/site-packages/tango/server.py", line 666 in server_init_hook
  File "/builds/[MASKED]/pytango/venv/lib/python3.9/site-packages/tango/server.py", line 1797 in tango_loop
  File "/builds/[MASKED]/pytango/venv/lib/python3.9/site-packages/tango/green.py", line 112 in run
  File "/builds/[MASKED]/pytango/venv/lib/python3.9/site-packages/tango/server.py", line 1803 in __server_run
  File "/builds/[MASKED]/pytango/venv/lib/python3.9/site-packages/tango/server.py", line 1937 in run
  File "/builds/[MASKED]/pytango/venv/lib/python3.9/site-packages/tango/test_context.py", line 456 in target
  File "/opt/python/cp39-cp39/lib/python3.9/threading.py", line 917 in run
  File "/opt/python/cp39-cp39/lib/python3.9/threading.py", line 980 in _bootstrap_inner
  File "/opt/python/cp39-cp39/lib/python3.9/threading.py", line 937 in _bootstrap
Thread 0x00007fc46a513740 (most recent call first):
  File "/opt/python/cp39-cp39/lib/python3.9/threading.py", line 316 in wait
  File "/opt/python/cp39-cp39/lib/python3.9/queue.py", line 180 in get
  File "/builds/[MASKED]/pytango/venv/lib/python3.9/site-packages/tango/test_context.py", line 559 in connect
  File "/builds/[MASKED]/pytango/venv/lib/python3.9/site-packages/tango/test_context.py", line 554 in start
  File "/builds/[MASKED]/pytango/venv/lib/python3.9/site-packages/tango/test_context.py", line 613 in __enter__
  File "/builds/[MASKED]/pytango/tests/test_server.py", line 3593 in test_server_init_hook_subscribe_event_multiple_devices
...

When trying to induce this fault running through a C++ debugger, it hasn't worked. Instead we should enable core dumps for Linux source job and then print out backtrace if core file is found.

Edited by Anton Joubert