Remove testing dependency on http download for radon dataset

[canyon289]

Describe the bug
Loading radon dataset is causing network failures in tests. We should remove this dependency anyway since we want our tests to be as isolated as possible.

https://github.com/arviz-devs/arviz/blob/main/arviz/tests/base_tests/test_stats.py#L607

To Reproduce
See CI pipeline here
https://dev.azure.com/ArviZ/ArviZ/_build/results?buildId=4434&view=logs&j=e9e5789a-ef7e-54d7-c525-bfdc535bcf80&t=85614d98-b825-523f-f666-d210d5fbfe6e&l=359

Reference PR where we first noticed this #1710

Expected behavior
We test def test_loo_pit_multi_lik(): without need for external radon dataset

Additional context

[canyon289]

@utkarsh-maheshwari assigning this to you for now so you have first right of refusal

[utkarsh-maheshwari]

I think task is to create inferencedata with multiple log_likelihood variables, without using any external datasets.
However, internal datsets like

arviz/arviz/tests/helpers.py

Line 181 in 4d9caca

model_1 = create_model(seed=10)

can be called inside test_loo_pit_multi_lik() and then adding to it an extra log_likelihood variable with random values would do our work.

[utkarsh-maheshwari]

Also, size of the new variable doesn't matter. It could be anything.

[utkarsh-maheshwari]

But I didn't actually understand the significance of this test.