Test for vtable mangling.

tests/run/mangle_c_keywords.pyx

+# Tests that illegal member and vtab entries are mangled.
+cdef class A:
+    """
+    >>> a = A(100)
+    >>> a.case()
+    100
+    """
+    def __init__(self, value):
+        self.switch = value
+    cdef int switch
+    cpdef case(self):
+        return self.switch