tests: Add basic tests for menu plugin

This checks basic features of kas menu, some option selection, yaml
including, rebuilding and target selection.

The UI is operated by mocking smack's GridFormHelp.runOnce and some
result evaluation functions. Injecting keys from the keyboard was not
easily possibly (if at all), thus this approach. The menu rendering is
not shown due to pytest.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

tests/test_menu/oe-init-build-env

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+export PATH=$(pwd):$PATH