Add unit test for nested cypclasses

tests/run/cypclass_nested_classes.pyx

+# mode: run
+# tag: cpp, cpp11
+# cython: experimental_cpp_class_def=True, language_level=2
+
+cdef cypclass A nolock:
+    int a
+
+    cypclass B nolock:
+        int b
+        __init__(self, int b):
+            self.b = b
+
+        int foo(self):
+            return self.b
+
+    B b
+
+    __init__(self, int a, int b):
+        self.a = a
+        self.b = B(b)
+    
+    int foo(self):
+        return self.a + self.b.foo()
+
+def test_nested_classes():
+    """
+    >>> test_nested_classes()
+    11
+    """
+    a = A(1, 10)
+    return a.foo()