feat: add util module with exists() function

tests/test_util.py

@@ -0,0 +1,14 @@
+# -*- coding: utf-8; -*-
+
+from unittest import TestCase
+from unittest.mock import MagicMock
+
+from wuttamess import util as mod
+
+
+class TestExists(TestCase):
+
+    def test_basic(self):
+        c = MagicMock()
+        mod.exists(c, '/foo')
+        c.run.assert_called_once_with('test -e /foo', warn=True)