Make @composite strategies collected by pytest act as failing tests

[Zac-HD]

From #1658, occasionally users use @composite when they should be using @given(st.data()). Unfortunately, this silently goes wrong:

from hypothesis.strategies import composite

@composite
def test_data_factory(draw):
    foo = draw(custom_objects)
    assert foo.check_bar()

Pytest will happily collect test_data_factory, execute it, and report it as a passing test... even if you replace the function body with assert False! This is because calling the function returns a LazyStrategy object, and the function isn't actually executed until later. More generally, it's because @composite should not be used to define test functions!

So it would be good to add a check to our pytest plugin, in pytest_collection_modifyitems, that replaces the collected item.obj with a function that will (a) fail, and (b) do so with a useful error suggesting that the user either rename the strategy or if intending to execute it as a test use @given(st.data()) instead.