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>

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')