Add proof-of-concept unittest

Doesn't do anything useful yet.

python/tests/test_mrmailbox.py

@@ -0,0 +1,16 @@
+import pathlib
+
+import pytest
+
+from deltachat import capi
+
+
+@pytest.fixture
+def tmppath(tmpdir):
+    return pathlib.Path(tmpdir.strpath)
+
+
+def test_new():
+    mbox = capi.lib.mrmailbox_new(capi.ffi.NULL, capi.ffi.NULL, capi.ffi.NULL)
+    capi.lib.mrmailbox_close(mbox)
+    assert 0