Add unit test against inplace operator with baseclass rhs inferring lhs to baseclass

tests/run/cypclass_inplace_operator_inference.pyx

+# mode: run
+# tag: cpp, cpp11
+# cython: experimental_cpp_class_def=True, language_level=2
+
+cdef cypclass A nolock:
+    int val
+    __init__(self, int a):
+        self.val = a
+    
+    A __iadd__(self, A other):
+        self.val += (other.val + 1)
+
+cdef cypclass B(A) nolock:
+    B __iadd__(self, A other):
+        self.val += (other.val + 10)
+
+    B __iadd__(self, B other):
+        self.val += (other.val + 100)
+
+def test_inplace_operator_inference():
+    """
+    >>> test_inplace_operator_inference()
+    111
+    """
+    a = A(0)
+    b0 = B(0)
+    b1 = B(0)
+
+    # at this point, the types are unambiguous and the following assignments should not cause them to infer as another type.
+
+    a += b0 # should add 1
+
+    # before it being fixed, 'b0 += a' where 'a' is of type A caused 'b0' to be inferred as type A instead of B.
+    
+    b0 += a # should add 10
+
+    # since all cypclass methods are virtual, 'b0' being erroneously inferred as type A would cause 
+    # 'b0 += b1' to call 'B __iadd__(self, A other)' instead of 'B __iadd__(self, B other)'.
+
+    b0 += b1 # should add 100
+
+    return b0.val