Fix broken test for models with batchnorm
One of the Keras tests assumed that fitting a model for one iteration with a learning rate of zero would not change any weights. This is not true for `BatchNorm`, which updates its running means and variances regardless! As a result, the model after the iteration had slightly different outputs, which caused the test to be very flaky. We now reinitialize the model after the single training epoch to make sure this doesn't happen.
Related to #17427, cc @amyeroberts .<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>🧠 <|||||>Give @amyeroberts the brain emoji for that one - she identified the whole problem, I just fixed the test!