[feature request] Global subtest function

[Conchylicultor]

Currently, when using subtest, one has to propagate the subtest object in each of the submethods, adding boilerplate:

def test_xyz(subtests):
  assert_fn(x, y, subtests)  # Propagate subtest to all sub functions


def assert_fn(x, y, subtests):
  with subtests.test('a'):
    with subtests.test('x'):
      assert_child_fn(x, subtests)  # Additional function with subtests
    with subtests.test('y'):
      assert_child_fn(x, subtests)

It would be nice if there was instead a global subtest function which could be called directly. The above code could be rewritten as

@pytest.mark.usefixture('subtest')
def test_xyz():
  assert_fn(x, y)  # No more subtest argument


def assert_fn(x, y):
  with pytest.subtest('a'):
    with pytest.subtest('x'):
      assert_child_fn(x)
    with pytest.subtest('y'):
      assert_child_fn(x)

Of course, calling pytest.subtests would raise an error if executed in a test which does not use the subtest fixture. But this would make it easier to make modular tests function.

If pytest.subtest is not possible, import subtests directly would be great too.