Port tests to pytest

Use simple asserts and pytest's powerful introspection to make testing
simpler. This merges the old .py/.ref file pairs into simple .py files
where the expected values are right next to the code being tested.

This commit does not touch the C++ part of the code and replicates the
Python tests exactly like the old .ref-file-based approach.

tests/test_eval.py

@@ -0,0 +1,22 @@
+
+
+def test_eval(capture):
+    from pybind11_tests import example_eval
+
+    with capture:
+        example_eval()
+    assert capture == """
+        eval_statements test
+        Hello World!
+        eval_statements passed
+        eval test
+        eval passed
+        eval_single_statement test
+        eval_single_statement passed
+        eval_file test
+        eval_file passed
+        eval failure test
+        eval failure test passed
+        eval_file failure test
+        eval_file failure test passed
+    """