Tidy functional testing

[wxtim]

A number of functional tests use the pattern:

@pytest.fixture
def workflow_creation_fixture:
    ...
    yield {...info...}

@pytest.fixture
def run_some_cylc_program(workflow_creation_fixture):
    ...
    yield {...more info..., 'workflow_creation_fixture': workflow_creation_fixture}

This would be better if the second fixture yielded:

    yield {...more info..., **workflow_creation_fixture}

to avoid unnecessary nesting.

Pull requests welcome!