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/doc/develop/pytest/index.rst b/doc/develop/pytest/index.rst
index 4d949cf..0a7c1b2 100644
--- a/doc/develop/pytest/index.rst
+++ b/doc/develop/pytest/index.rst
@@ -21,6 +21,7 @@
    test_bind
    test_bootmenu
    test_bootstage
+   test_button
    test_efi_loader
    test_net
    test_net_boot