Move tests out of app module

(Directory structure as suggested in
[Django testing docs][1].)

[1]: https://docs.djangoproject.com/en/1.9/topics/testing/advanced/#using-the-django-test-runner-to-test-reusable-applications

tests/utils.py

@@ -0,0 +1,47 @@
+# Anymail test utils
+import os
+import unittest
+from base64 import b64decode
+
+import six
+
+
+def decode_att(att):
+    """Returns the original data from base64-encoded attachment content"""
+    return b64decode(att.encode('ascii'))
+
+
+SAMPLE_IMAGE_FILENAME = "sample_image.png"
+
+
+def sample_image_path(filename=SAMPLE_IMAGE_FILENAME):
+    """Returns path to an actual image file in the tests directory"""
+    test_dir = os.path.dirname(os.path.abspath(__file__))
+    path = os.path.join(test_dir, filename)
+    return path
+
+
+def sample_image_content(filename=SAMPLE_IMAGE_FILENAME):
+    """Returns contents of an actual image file from the tests directory"""
+    filename = sample_image_path(filename)
+    with open(filename, "rb") as f:
+        return f.read()
+
+
+class AnymailTestMixin:
+    """Helpful additional methods for Anymail tests"""
+
+    pass
+    # Plus these methods added below:
+    # assertCountEqual
+    # assertRaisesRegex
+    # assertRegex
+
+# Add the Python 3 TestCase assertions, if they're not already there.
+# (The six implementations cause infinite recursion if installed on
+# a py3 TestCase.)
+for method in ('assertCountEqual', 'assertRaisesRegex', 'assertRegex'):
+    try:
+        getattr(unittest.TestCase, method)
+    except AttributeError:
+        setattr(AnymailTestMixin, method, getattr(six, method))