Docs: adding explicit mention of test_utils to docs (#11670)

* Docs: adding explicit mention of test_utils to docs

* Improved wording

Author: edmondop

docs/source/contributor-guide/testing.md

@@ -30,6 +30,7 @@ and tries to follow the Rust standard [Testing Organization](https://doc.rust-la
 ## Unit tests
 
 Tests for code in an individual module are defined in the same source file with a `test` module, following Rust convention.
+The [test_util](https://github.com/apache/datafusion/tree/main/datafusion/common/src/test_util.rs) module provides useful macros to write unit tests effectively, such as `assert_batches_sorted_eq` and `assert_batches_eq` for RecordBatches and `assert_contains` / `assert_not_contains` which are used extensively in the codebase.
 
 ## sqllogictests Tests