Test for `NegativeBinomialNoise`

[dylanhmorris]

Not for the PR: a test or NegativeBinomialNoise as well.

Originally posted by @dylanhmorris in #644 (comment)

[cdc-mitzimorris]

Closed: NegativeBinomialNoise is exercised in test/test_observation_counts.py

• test_negative_binomial_observation (sampling variability),
• test_with_stochastic_ascertainment
• test_with_stochastic_concentration (priors)
• two validate_concentration_rv tests covering zero and negative concentration.