doc: pytest: Document the test_button test

Add this test to the documentation. No changes to the test itself were
required.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/py/tests/test_button.py b/test/py/tests/test_button.py
index f0d85be..f3f6816 100644
--- a/test/py/tests/test_button.py
+++ b/test/py/tests/test_button.py
@@ -1,5 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0+
 
+"""Tests for the button command"""
+
 import pytest
 
 @pytest.mark.boardspec('sandbox')