Update tests on `poisson_reg()`

[hfrick]

This PR updates the tests on poisson_reg() to stay in sync with the testing pattern in parsnip and other extensions:

• tests on a model spec that can be run without an engine go into parsnip (because the model definition lives there)
• tests on the model spec that do need an engine go into the package where the engine lives (here: poissonreg)

The changes are easier to read by commits: 1) The first commit removes tests that are moved to parsnip in https://github.com/tidymodels/parsnip/pull/854. 2) The second commit updates tests on model arguments which do require an engine to the current testing pattern in parsnip and other extensions.

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