Add tests to verify that sampler stats match the advertised `stats_dtypes`

[michaelosthege]

Every step method has a stats_dtypes dictionary that advertises the dtypes to be expected.

This information is relevant for storage backends so they can create arrays of the correct dtypes.

In #5882 a fix was needed because previous changes to the pm.Metropolis stepper did not update the stats_dtypes dictionary.

We already have tests that are parametrized by step methods, so instead of creating a new test for this issue, we should try to add the asserts in there.