Add some tests

Author: DilumAluthge

test/examples/multinomial_logistic_regression.jl

@@ -39,6 +39,8 @@ Test.@test MaximumLikelihoodProblems.Internal._is_approximately_zero(gradient_ve
 Test.@test MaximumLikelihoodProblems.Internal._is_approximately_hermitian(hessian_matrix_at_θ_hat)
 Test.@test MaximumLikelihoodProblems.Internal._is_approximately_negative_definite(hessian_matrix_at_θ_hat;
                                                                                   fuzz_factor = 10)
+Test.@test MaximumLikelihoodProblems.Internal._is_approximately_positive_definite(-hessian_matrix_at_θ_hat;
+                                                                                  fuzz_factor = 10)
 
 external_df = DataFrames.DataFrame()
 external_df[!, :X_2] = X[:, 2]