Fixed asserting that no warnings were raised

[radarhere]

The following test is intended to check that no warnings are raised when opening and closing an image.

Pillow/Tests/test_file_fli.py

Lines 40 to 46 in cc4ed21

	def test_closed_file():
	def open():
	im = Image.open(static_test_file)
	im.load()
	im.close()
	pytest.warns(None, open)

If I change it like so

def test_closed_file():
    def open():
        Image.open(static_test_file)

    pytest.warns(ResourceWarning, open)

It passes (except in PyPy). So we know that a ResourceWarning is raised by my modification.

But if I change it back to assert that there are no warnings,

def test_closed_file():
    def open():
        Image.open(static_test_file)

    pytest.warns(None, open)

It still passes.

This is because we are not using pytest correctly.

https://docs.pytest.org/en/stable/warnings.html

To record with func:pytest.warns without asserting anything about the warnings, pass None as the expected warning type:

with pytest.warns(None) as record:
   warnings.warn("user", UserWarning)
   warnings.warn("runtime", RuntimeWarning)

assert len(record) == 2

This PR corrects our usage in various tests.

[nulano]

From https://docs.pytest.org/en/stable/warnings.html#custom-failure-messages:

If no warnings are issued when calling f, then not record will evaluate to True.

Would it be clearer to use assert not record instead?

[radarhere]

Ok, I've pushed another commit for that change

[hugovk]

Good catch, thanks!