Add unit test against ambiguously generated C++ calls of overloaded methods

tests/run/cpp_ambiguous_overloading.pyx

+# mode: run
+# tag: cpp, cpp11
+# cython: experimental_cpp_class_def=True, language_level=2
+
+cdef cppclass A:
+    int foo(int a = 0):
+        # always return odd result
+        return 2 * a + 1
+
+    int foo(int* a):
+        # always return even result
+        if a:
+            return 2 * a[0]
+        return 0
+
+def test_ambiguous_overloading():
+    """
+    >>> test_ambiguous_overloading()
+    1
+    """
+    b = new A()
+
+    # This turns into a C++ call to the first "foo" method with a NULL argument 
+    # (no argument passed to a method with default-value parameter)
+    # This "NULL" needs to be correctly cast, otherwise the call is ambiguous:
+    # it could be the second "foo" method with a NULL pointer.
+    # In that case the generated C++ would not compile.
+    r = b.foo()
+
+    # Check that the correct "foo" method was called and returned expected result.
+    return r