Tests: move core tests to pytest

Use fixture workaround to be able to run the test with 4 configurations
Some cleanup might still be needed but this is a first attempt to migrate this part of the tests

tests/conftest.py

@@ -0,0 +1,18 @@
+# Copyright 2014-2017 Insight Software Consortium.
+# Copyright 2004-2009 Roman Yakovenko.
+# Distributed under the Boost Software License, Version 1.0.
+# See http://www.boost.org/LICENSE_1_0.txt
+
+import pytest
+
+
+class Helpers:
+    @staticmethod
+    def _test_type_composition(type_, expected_compound, expected_base):
+        assert isinstance(type_, expected_compound)
+        assert isinstance(type_.base, expected_base)
+
+
+@pytest.fixture
+def helpers():
+    return Helpers