Speedup slow CI tests

[ricardoV94]

The setup for different parametrizations of tests/mmm/test_plotting.py::TestBasePlotting is taking 12 minutes (nearly 1/3) of the CI. I don't know what it is doing, but that's unreasonable :)

If some fitting is being done, we should replace it by dummy draws.

https://github.com/pymc-labs/pymc-marketing/actions/runs/5900612302/job/16005192098#step:5:548

tests/mmm/test_delayed_saturated_mmm.py::TestDelayedSaturatedMMM::test_channel_contributions_forward_pass_recovers_contribution also seems too slow. If a function is being compiled multiple times we should cache it (#314). Otherwise, we should try FAST_COMPILE or running it in python (not C) backend.

There are also several CLV... test_load_save that are way too slow. Again we shouldn't be doing costly fitting to test this functionality.

Example: https://github.com/pymc-labs/pymc-marketing/actions/runs/5900612302/job/16005192098#step:5:555

CC @juanitorduz @michaelraczycki