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.

Resolve TMSS-652 and TMSS-714

Closes TMSS-652

See merge request !403

Resolve TMSS-678

Closes TMSS-678

See merge request !415

TMSS-652: processed review comment: moved magic truncation into descriptive function + link to (minimal) postgres documentation

Resolve TMSS-637

Closes TMSS-637

See merge request !412

TMSS-725: fixed tests, and added convenience method to set a subtask to a desired state following the allowed transitions.

TMSS-726: Fix _get_lta_dataproducts query; fix subtask creation in test

Closes TMSS-726

See merge request !413

Resolve TMSS-692

Closes TMSS-692

See merge request !410

TMSS-725: added a SubtaskAllowedStateTransitions table, populate it with the allowed state transitions, and raise an exception if a user tries to submit an illegal state transition

(cherry picked from commit 59e7460b)

(cherry picked from commit e24e22f3)

(cherry picked from commit 0f25f542)