doc: release-notes: add a note about tests & samples

Mention the ztest api conversion and new tests expectations.

Signed-off-by: Fabio Baltieri <[email protected]>

Author: fabiobaltieri

doc/releases/release-notes-3.2.rst

@@ -861,6 +861,10 @@ Documentation
 Tests and Samples
 *****************
 
+* A large number of tests have been reworked to use the new ztest API, see
+  :ref:`test-framework` for more details. This should be used for newly
+  introduce tests as well.
+
 Issue Related Items
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