kconfig: test: add Kconfig macro language tests

Here are the test cases I used for developing the text expansion
feature.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/tests/preprocess/variable/__init__.py

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: GPL-2.0
+"""
+Variable and user-defined function tests.
+"""
+
+def test(conf):
+    assert conf.oldaskconfig() == 0
+    assert conf.stderr_matches('expected_stderr')