TMSS-666: TMSS-481 TODO: fix test, there are issues with permissions since we changed the method from GET to POST

TMSS-666: removed set_subtask_state_following_allowed_transitions from subtasks.py. It was moved in TMSS-725 to test_utils.py

TMSS-613: Beamformer SAP subbands issue solved. Added clear button in CustomDate component . Updated few messages.

TMSS-725: processed review comment: split the ERROR state into (generic)ERROR and UNSCHEDULABLE which are both end-states.

TMSS-725: processed review comment. moved set_subtask_state_following_allowed_transitions into test_utils so that it is clearly intended to be used in tests only, and not in normal tmss code. Took the opportunity to move the TMSSTestEnvironment into its own module as well to make a clear distinction between test-helper-methods (in test_utils.py) and the TMSSTestEnvironment (in test_environment.py). Adapted all tests with new import paths.