Add settable property test

Author: hamdanal

mypy/test/teststubtest.py

@@ -918,13 +918,15 @@ class Z:
                 def read_write_attr(self) -> int: ...
                 @read_write_attr.setter
                 def read_write_attr(self, val: int) -> None: ...
+                read_write_attr_alias = read_write_attr
             """,
             runtime="""
             class Z:
                 @property
                 def read_write_attr(self): return self._val
                 @read_write_attr.setter
                 def read_write_attr(self, val): self._val = val
+                read_write_attr_alias = read_write_attr
             """,
             error=None,
         )