add assert

Signed-off-by: yaommen <myanstu@163.com>

Author: myandpr

datafusion/expr/src/type_coercion/functions.rs

@@ -1243,6 +1243,7 @@ mod tests {
         );
 
         let err = fields_with_udf(&current_fields, &MockUdf(signature)).unwrap_err();
+        assert!(matches!(err, DataFusionError::Plan(_)));
         let err = err.to_string();
 
         assert!(err.starts_with(
@@ -1261,6 +1262,7 @@ mod tests {
         );
 
         let err = fields_with_udf(&current_fields, &MockUdf(signature)).unwrap_err();
+        assert!(matches!(err, DataFusionError::Plan(_)));
         let err = err.to_string();
 
         assert!(err.starts_with(
@@ -1293,6 +1295,7 @@ mod tests {
 
         let err =
             fields_with_udf(&current_fields, &AlwaysExecErrUdf(signature)).unwrap_err();
+        assert!(matches!(err, DataFusionError::Execution(_)));
 
         assert!(err.to_string().starts_with(
             "Execution error: Function 'test' user-defined coercion failed with: Execution error: boom"