Test scripts use different ways to define the default Test Event Trigger Key

In the current version of the Test Scripts there are two different ways to define the default Test Event Trigger Key:

1. 0x000102030405060708090a0b0c0d0e0f
   This is used by the function send_test_event_triggers() in src/python_testing/matter_testing_support.py which is always called without the key parameter. Thus the default key 0x000102030405060708090a0b0c0d0e0f gets used or the value of the global parameter enableKey if it exists

2. 0x00112233445566778899aabbccddeeff
   This is hard-coded as a default in various YAML scripts like for DGGEN-2.1, DD-3.2, DD-3.6, DD-3.8, etc.

IMHO we should at least unify the hard-coded default keys to one value for all scripts to avoid confusion on the tester side.