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_modules.py

@@ -0,0 +1,56 @@
+
+def test_nested_modules(capture):
+    import pybind11_tests
+    from pybind11_tests.submodule import submodule_func
+
+    assert pybind11_tests.__name__ == "pybind11_tests"
+    assert pybind11_tests.submodule.__name__ == "pybind11_tests.submodule"
+
+    with capture:
+        submodule_func()
+    assert capture == "submodule_func()"
+
+
+def test_reference_internal():
+    from pybind11_tests import ConstructorStats
+    from pybind11_tests.submodule import A, B
+
+    b = B()
+    assert str(b.get_a1()) == "A[1]"
+    assert str(b.a1) == "A[1]"
+    assert str(b.get_a2()) == "A[2]"
+    assert str(b.a2) == "A[2]"
+
+    b.a1 = A(42)
+    b.a2 = A(43)
+    assert str(b.get_a1()) == "A[42]"
+    assert str(b.a1) == "A[42]"
+    assert str(b.get_a2()) == "A[43]"
+    assert str(b.a2) == "A[43]"
+
+    astats, bstats = ConstructorStats.get(A), ConstructorStats.get(B)
+    assert astats.alive() == 2
+    assert bstats.alive() == 1
+    del b
+    assert astats.alive() == 0
+    assert bstats.alive() == 0
+    assert astats.values() == ['1', '2', '42', '43']
+    assert bstats.values() == []
+    assert astats.default_constructions == 0
+    assert bstats.default_constructions == 1
+    assert astats.copy_constructions == 0
+    assert bstats.copy_constructions == 0
+    # assert astats.move_constructions >= 0  # Don't invoke any
+    # assert bstats.move_constructions >= 0  # Don't invoke any
+    assert astats.copy_assignments == 2
+    assert bstats.copy_assignments == 0
+    assert astats.move_assignments == 0
+    assert bstats.move_assignments == 0
+
+
+def test_importing():
+    from pybind11_tests import OD
+    from collections import OrderedDict
+
+    assert OD is OrderedDict
+    assert str(OD([(1, 'a'), (2, 'b')])) == "OrderedDict([(1, 'a'), (2, 'b')])"