mock posterior with prior for MMM tests

[wd60622]

Follow up from #515 and #514

Goal:

• don't call fit but use a mocked posterior for testing majority of the methods
• call fit and check for convergence and mark test as slow

[ricardoV94]

This is still relevant, we are spending 5 minutes (25% CI) setting up the fixtures for this test, just to check nothing crashes when we call the plot utilities: https://github.com/pymc-labs/pymc-marketing/actions/runs/8139313836/job/22242195431?pr=565#step:5:827

[wd60622]

I am going to pick up this issue

[wd60622]

Some were address. However others need to be changed as well tests for actual fits with mark of slow