Expression tests: Fix arguments for beta_neg_binomial_rng

Author: WardBrian

test/sig_utils.py

@@ -75,6 +75,7 @@ def get_cpp_type(stan_type):
     "acosh": [1.4],
     "algebra_solver": [None, None, None, None, None, None, None, 10],
     "algebra_solver_newton": [None, None, None, None, None, None, None, 10],
+    "beta_neg_binomial_rng": [1.1, 3.1, 8.1],
     "log1m_exp": [-0.6],
     "categorical_rng": [simplex, None],
     "categorical_lpmf": [None, simplex],