Added serde feature flags to tests

src/tree/decision_tree_classifier.rs

@@ -745,6 +745,7 @@ mod tests {
     }
 
     #[test]
+    #[cfg(feature = "serde")]
     fn serde() {
         let x = DenseMatrix::from_2d_array(&[
             &[1., 1., 1., 0.],

src/tree/decision_tree_regressor.rs

@@ -581,6 +581,7 @@ mod tests {
     }
 
     #[test]
+    #[cfg(feature = "serde")]
     fn serde() {
         let x = DenseMatrix::from_2d_array(&[
             &[234.289, 235.6, 159., 107.608, 1947., 60.323],