Fix flaky integration tests

[MetRonnie]

Fixes state leakage between integration tests that was resulting in tests/integration/reftests/test_cyclers.py::test_icp_fcp_notation failing flakily.

Salient traceback

cylc/flow/scheduler.py:744: in start
    await self.handle_exception(exc)
cylc/flow/scheduler.py:1996: in handle_exception
    raise exc from None
cylc/flow/scheduler.py:741: in start
    await self.configure(params)
cylc/flow/scheduler.py:495: in configure
    self._load_pool_from_point()
cylc/flow/scheduler.py:782: in _load_pool_from_point
    self.pool.load_from_point()
cylc/flow/task_pool.py:197: in load_from_point
    self.spawn_to_rh_limit(tdef, point, {flow_num})
cylc/flow/task_pool.py:787: in spawn_to_rh_limit
    self.get_or_spawn_task(
cylc/flow/task_pool.py:745: in get_or_spawn_task
    ntask = self.spawn_task(tdef.name, point, flow_nums, flow_wait)
cylc/flow/task_pool.py:1686: in spawn_task
    itask = self._get_task_proxy_db_outputs(
cylc/flow/task_pool.py:1793: in _get_task_proxy_db_outputs
    itask = TaskProxy(
cylc/flow/task_proxy.py:294: in __init__
    self.graph_children = generate_graph_children(tdef, self.point)
cylc/flow/taskdef.py:49: in generate_graph_children
    if is_abs and trigger.get_parent_point(point) != point:
cylc/flow/task_trigger.py:74: in get_parent_point
    point = get_point(self.cycle_point_offset).standardise()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = +P0

    def standardise(self):
        """Reformat self.value into a standard representation."""
        try:
            self.value = str(point_parse(self.value))
        except IsodatetimeError as exc:
            if self.value.startswith("+") or self.value.startswith("-"):
                message = WARNING_PARSE_EXPANDED_YEAR_DIGITS % (
                    WorkflowSpecifics.NUM_EXPANDED_YEAR_DIGITS)
            else:
                message = str(exc)
>           raise PointParsingError(type(self), self.value, message)
E           cylc.flow.exceptions.PointParsingError: Incompatible value for <class 'cylc.flow.cycling.iso8601.ISO8601Point'>: +P0: (incompatible with [cylc]cycle point num expanded year digits = 0 ?)

cylc/flow/cycling/iso8601.py:105: PointParsingError

Also it was failing in a confusing way, so I altered the integration reftest function to better handle unexpected shutdown

[MetRonnie]

MacOS tests failing for unrelated reasons; not tackled in this PR

[MetRonnie]

This fixture was waiting for 60 secs for the workflow to shut down even if there was no contact data. If there is no contact data it must have already shut down due to an error or something (which bypasses the set_stop method).

[wxtim]

it might be overloaded, but I think await_tokens would be better English.

[wxtim]

Nice -One entirely subjective comment about naming which you can take or leave.

[MetRonnie]

MacOS integration test failure is a flaky TUI test

[oliver-sanders]

(will leave you to accept/reject Tim's suggestion and merge)