Add test cases for yule-simon cdf

also check the lower tail

Author: lingium

test/prob/yule_simon/yule_simon_cdf_test.hpp

@@ -19,6 +19,16 @@ class AgradCdfYuleSimon : public AgradCdfTest {
     param[1] = 5.5;  // alpha
     parameters.push_back(param);
     cdf.push_back(0.9997987132162779);  // expected cdf
+
+    param[0] = 1;   // n
+    param[1] = 0.1;  // alpha
+    parameters.push_back(param);
+    cdf.push_back(0.0909090909090918);  // expected cdf
+
+    param[0] = 1;   // n
+    param[1] = 0.01;  // alpha
+    parameters.push_back(param);
+    cdf.push_back(0.0099009900990106);  // expected cdf
   }
 
   void invalid_values(vector<size_t>& index, vector<double>& value) {