Add test for gamma_cdf_func to validate behavior for negative input

src/compute/stats/distributions.rs

@@ -360,6 +360,7 @@ mod tests {
 
         // <0 case
         assert_eq!(gamma_pdf_func(-1.0, 1.0, 1.0), 0.0);
+        assert_eq!(gamma_cdf_func(-1.0, 1.0, 1.0), 0.0);
     }
     #[test]
     fn test_gamma_matrix() {
@@ -376,11 +377,6 @@ mod tests {
         let x = Matrix::filled(5, 5, 1.0);
         let expected = lower_incomplete_gamma_func(2.0, 1.0);
         let result = lower_incomplete_gamma(s, x);
-        assert!(
-            (result.data()[0] - expected).abs() < 1e-7,
-            "Expected: {}, Got: {}",
-            expected,
-            result.data()[0]
-        );
+        assert!((result.data()[0] - expected).abs() < 1e-7);
     }
 }