[Backport 1.30] CI: Pins pytest to 8.3.4 for Test core addons (#4923)

[louiseschmidtgen]

Currently, the "Test core addons" job fails because the test_cis test fails, having 2 more warnings than expected.

Those warnings are from microk8s kube-burner, which has the following new warnings (the checks were passing before):

[WARN] 2.3 Ensure that the --auto-tls argument is not set to true (Automated)
[WARN] 2.6 Ensure that the --peer-auto-tls argument is not set to true (Automated)
``

Those warnings appear because the commands used for checking them fail:

2.3 audit test did not run: failed to run: "/bin/ps -ef | /bin/grep k8s-dqlite | /bin/grep -v grep", output: "", error: exit status 1 2.6 audit test did not run: failed to run: "/bin/ps -ef | /bin/grep k8s-dqlite | /bin/grep -v grep", output: "", error: exit status 1

Those errors would suggest that there is no ``k8s-dqlite`` process running
on the node... but it should. In ``test_cis``, after enabling
``cis-hardening`` (``microk8s enable cis-hardening``) and before running
``microk8s kube-burner``, in the test we're waiting for Kubernetes to be
available, accesible, and ready (``wait_for_installation``), which would only
be possible if ``k8s-dqlite`` would be running. What's more, this issue
does not occur when manually running the exact same commands as in the
GitHub action, even on the same GitHub Runners.

Pinning pytest to 8.3.4 seems to be solving the issue above (8.3.5 was released
a few days ago).