docstr: ...Test PoissonRegression decision function
          
docstr_tok: ["...","Test","PoissonRegression","decision","function"]
code: def test_decision_function(self):
      """...Test PoissonRegression decision function"""
      X_train, y_train, _, _ = self.get_train_data(
          n_samples=200, n_features=12
      )
      learner = PoissonRegression(random_state=32789, tol=1e-9)
      learner.fit(X_train, y_train)
      X_test, y_test, _, _ = self.get_train_data(
          n_samples=5, n_features=12
      )
      y_pred = np.array([1.1448, 5.2194, 0.2624, 4.5525, 6.4168])
      np.testing.assert_array_almost_equal(
          learner.decision_function(X_test), y_pred, decimal=4
      )
  
imports: import unittest
  from itertools import product
  import numpy
  from tick.base.inference import InferenceTest
  from tick.linear_model import SimuPoisReg, PoissonRegression
  from tick.simulation import weights_sparse_gauss
label_desc: Use image node_id 14 for calling the Test obj's underlying member method code with example usage: obj.test_decision_function() without return types