bt2: ignore comparison to None flake8 warnings in test_value.py

Fix these:

    ./tests/bindings/python/bt2/test_value.py:485:36: E711 comparison to None should be 'if cond is None:'
    ./tests/bindings/python/bt2/test_value.py:488:35: E711 comparison to None should be 'if cond is not None:'
    ./tests/bindings/python/bt2/test_value.py:1177:36: E711 comparison to None should be 'if cond is None:'
    ./tests/bindings/python/bt2/test_value.py:1180:35: E711 comparison to None should be 'if cond is not None:'

In these tests, we specifically want to test that comparing (equal/not
equal) to None yields the expected result.

Reported-by: flake8
Change-Id: I47d96598a158b10116fc6a948d42ef61f608fb07
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2087