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_stl_binders.py

@@ -0,0 +1,53 @@
+
+
+def test_vector_int():
+    from pybind11_tests import VectorInt
+
+    v_int = VectorInt([0, 0])
+    assert len(v_int) == 2
+    assert bool(v_int) is True
+
+    v_int2 = VectorInt([0, 0])
+    assert v_int == v_int2
+    v_int2[1] = 1
+    assert v_int != v_int2
+
+    v_int2.append(2)
+    v_int2.append(3)
+    v_int2.insert(0, 1)
+    v_int2.insert(0, 2)
+    v_int2.insert(0, 3)
+    assert str(v_int2) == "VectorInt[3, 2, 1, 0, 1, 2, 3]"
+
+    v_int.append(99)
+    v_int2[2:-2] = v_int
+    assert v_int2 == VectorInt([3, 2, 0, 0, 99, 2, 3])
+    del v_int2[1:3]
+    assert v_int2 == VectorInt([3, 0, 99, 2, 3])
+    del v_int2[0]
+    assert v_int2 == VectorInt([0, 99, 2, 3])
+
+
+def test_vector_custom():
+    from pybind11_tests import El, VectorEl, VectorVectorEl
+
+    v_a = VectorEl()
+    v_a.append(El(1))
+    v_a.append(El(2))
+    assert str(v_a) == "VectorEl[El{1}, El{2}]"
+
+    vv_a = VectorVectorEl()
+    vv_a.append(v_a)
+    vv_b = vv_a[0]
+    assert str(vv_b) == "VectorEl[El{1}, El{2}]"
+
+
+def test_vector_bool():
+    from pybind11_tests import VectorBool
+
+    vv_c = VectorBool()
+    for i in range(10):
+        vv_c.append(i % 2 == 0)
+    for i in range(10):
+        assert vv_c[i] == (i % 2 == 0)
+    assert str(vv_c) == "VectorBool[1, 0, 1, 0, 1, 0, 1, 0, 1, 0]"