Alternatives created by fixture unions should be easily visible in test ids

[smarie]

from pytest_cases import param_fixture, fixture_union

a = param_fixture("a", [1, 2])
b = param_fixture("b", [3, 4])

c = fixture_union('c', ['a', 'b'])

def test_the_ids(c):
    pass

Currently leads to the following ids:

test_fixture_union_ids.py::test_the_ids[a-1] 
test_fixture_union_ids.py::test_the_ids[a-2] 
test_fixture_union_ids.py::test_the_ids[b-3] 
test_fixture_union_ids.py::test_the_ids[b-4]

It will maybe not be very understandable in more complex cases. Besides as of today if one tries to set the ids manually he will get an error ValueError: ids cannot be set on a union fixture.

I propose to (1) allow ids to be set on union fixtures, and (2) propose an additional "idstyle" argument with three options:

• idstyle=None: a, b (current implementation): there is no way to distinguish this from other parameters
• idstyle='compact': Ua, Ub
• idstyle='explicit': c_is_a, c_is_b. This would become the default.

Note that this new parameter should be available in pytest_parametrize_plus too.