Have more coherent testing of the `summary` method

[drbenvincent]

At the moment it seems that we only test the summary method for the pymc_experiments.DifferenceInDifferences class, and that is in the test_pymc_experiments.py file...

https://github.com/pymc-labs/CausalPy/blob/3bb81a702a19e725b1b180b8ddbc1aa3e453204c/causalpy/tests/test_pymc_experiments.py#L10-L23

We should make sure that we are testing the summary method for all of the pymc_experiment classes. Though I believe the InstrumentalVariable class doesn't have a summery method.

This could be done by adding more to the current test_pymc_experiments.py file, but it could be worth thinking about simply adding to existing tests in test_integration_pymc_examples.py. My thinking is that this would involve less setup / computation time as we wouldn't be doing any additional model fitting.