[tests] Update DeprecatorTestCase.test_deprecated_function_docstring
The multiline docstring behaviour was change in Python 3.13:
https://docs.python.org/3.13/whatsnew/3.13.html#other-language-changes
Add a new test mehtod test_deprecated_function_multiline_docstring to
take into accoutn that there is a difference.
Bug: T359069
Change-Id: I7aa1f53fc8612bfe6f40ad098d2dcbb6d3d74c80