upgrade tests are flaky

[beliaev-maksim]

it looks like that when we call for an upgrade sometimes we have a race when the charm is refreshed but manifests are still of the old version.
That causes tests to fail (which is healed by rerun)

Traceback (most recent call last):
  File "/home/runner/work/training-operator/training-operator/tests/integration/test_charm.py", line 261, in test_upgrade
    assert (
AssertionError: assert ('tfjobs.kubeflow.org', 'v0.6.0') in [('xgboostjobs.kubeflow.org', 'v0.10.0'), ('tfjobs.kubeflow.org', 'v0.10.0'), ('pytorchjobs.kubeflow.org', 'v0.10.0'), ('mxjobs.kubeflow.org', 'v0.10.0'), ('mpijobs.kubeflow.org', 'v0.10.0'), ('paddlejobs.kubeflow.org', 'v0.10.0')]

see https://github.com/canonical/training-operator/actions/runs/5331136179/jobs/9660014634