relax chromaticity test for numpy 1.24

[lfarv]

The linopt6 test suddenly started failing on Ubuntu. This is correlated with the release of numpy 1.24. The same code (stub branch) was running before merging:

Run python -m pytest test --cov-report term-missing --cov=at
============================= test session starts ==============================
platform linux -- Python 3.11.1, pytest-7.2.0, pluggy-1.0.0
system: Linux 5.15.0-1024-azure
numpy version: 1.23.5
rootdir: /home/runner/work/at/at
plugins: lazy-fixture-0.6.3, cov-4.0.0
collected 295 items

But now fails with a new numpy version:

Run python -m pytest test --cov-report term-missing --cov=at
============================= test session starts ==============================
platform linux -- Python 3.11.1, pytest-7.2.0, pluggy-1.0.0
system: Linux 5.15.0-1024-azure
numpy version: 1.24.0
rootdir: /home/runner/work/at/at
plugins: lazy-fixture-0.6.3, cov-4.0.0
collected 295 items

The chromaticity test between linopt2 and linopt6 must be relaxed from atol=1.e-12 to atol=5.e-11.
The accuracy is still good enough, but it's worrying that results may change with time…

The problem seems to be specific to Ubuntu

[lfarv]

Merged to unlock the tests in other branches. !