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_operator_overloading.py

@@ -0,0 +1,41 @@
+
+def test_operator_overloading():
+    from pybind11_tests import Vector2, Vector, ConstructorStats
+
+    v1 = Vector2(1, 2)
+    v2 = Vector(3, -1)
+    assert str(v1) == "[1.000000, 2.000000]"
+    assert str(v2) == "[3.000000, -1.000000]"
+
+    assert str(v1 + v2) == "[4.000000, 1.000000]"
+    assert str(v1 - v2) == "[-2.000000, 3.000000]"
+    assert str(v1 - 8) == "[-7.000000, -6.000000]"
+    assert str(v1 + 8) == "[9.000000, 10.000000]"
+    assert str(v1 * 8) == "[8.000000, 16.000000]"
+    assert str(v1 / 8) == "[0.125000, 0.250000]"
+    assert str(8 - v1) == "[7.000000, 6.000000]"
+    assert str(8 + v1) == "[9.000000, 10.000000]"
+    assert str(8 * v1) == "[8.000000, 16.000000]"
+    assert str(8 / v1) == "[8.000000, 4.000000]"
+
+    v1 += v2
+    v1 *= 2
+    assert str(v1) == "[8.000000, 2.000000]"
+
+    cstats = ConstructorStats.get(Vector2)
+    assert cstats.alive() == 2
+    del v1
+    assert cstats.alive() == 1
+    del v2
+    assert cstats.alive() == 0
+    assert cstats.values() == ['[1.000000, 2.000000]', '[3.000000, -1.000000]',
+                               '[4.000000, 1.000000]', '[-2.000000, 3.000000]',
+                               '[-7.000000, -6.000000]', '[9.000000, 10.000000]',
+                               '[8.000000, 16.000000]', '[0.125000, 0.250000]',
+                               '[7.000000, 6.000000]', '[9.000000, 10.000000]',
+                               '[8.000000, 16.000000]', '[8.000000, 4.000000]']
+    assert cstats.default_constructions == 0
+    assert cstats.copy_constructions == 0
+    assert cstats.move_constructions >= 10
+    assert cstats.copy_assignments == 0
+    assert cstats.move_assignments == 0