Add check for changed password

These tests are incomplete, but I have no idea how to write the other
half.

src/documents/tests/test_checks.py

@@ -0,0 +1,25 @@
+import unittest
+
+from django.test import TestCase
+
+from ..checks import changed_password_check
+from ..models import Document
+from .factories import DocumentFactory
+
+
+class ChecksTestCase(TestCase):
+
+    def test_changed_password_check_empty_db(self):
+        self.assertEqual(changed_password_check(None), [])
+
+    def test_changed_password_check_no_encryption(self):
+        DocumentFactory.create(storage_type=Document.STORAGE_TYPE_UNENCRYPTED)
+        self.assertEqual(changed_password_check(None), [])
+
+    @unittest.skip("I don't know how to test this")
+    def test_changed_password_check_gpg_encryption_with_good_password(self):
+        pass
+
+    @unittest.skip("I don't know how to test this")
+    def test_changed_password_check_fail(self):
+        pass