Tests without fixtures can cause false deadfixture positives

[tawonga]

Nice plugin.

I stumbled across a glitch in --deadfixtures. If a test which does not use fixtures is found, then the fixtures used by subsequent tests are considered dead.

This test fails

def test_dont_list_fixture_used_after_test_which_does_not_use_fixtures(testdir, message_template):
    testdir.makepyfile("""
        import pytest
        @pytest.fixture()
        def same_file_fixture():
            return 1
            
        def test_no_fixture_used():
            assert True
        def test_simple(same_file_fixture):
            assert 1 == same_file_fixture
    """)

    result = testdir.runpytest('--dead-fixtures')
    message = message_template.format(
        'same_file_fixture',
        'est_dont_list_fixture_used_after_test_which_does_not_use_fixtures'
    )

    assert result.ret == 0
assert message not in result.stdout.str()

The fix seems simple and I will raise a pull request for your consideration