gh-101100: Fix sphinx warnings in library/unittest.rst

[LamentXU123]

unittest.rst:438: WARNING: py:meth reference target not found: TestCase.__init__ [ref.meth]
unittest.rst:520: WARNING: py:class reference target not found: DocTestSuite [ref.class]
unittest.rst:1025: WARNING: py:attr reference target not found: exception [ref.attr]
unittest.rst:1039: WARNING: py:attr reference target not found: exception [ref.attr]
unittest.rst:1091: WARNING: py:attr reference target not found: warning [ref.attr]
unittest.rst:1091: WARNING: py:attr reference target not found: filename [ref.attr]
unittest.rst:1091: WARNING: py:attr reference target not found: lineno [ref.attr]
unittest.rst:1440: WARNING: py:meth reference target not found: set.difference [ref.meth]
unittest.rst:1648: WARNING: py:meth reference target not found: setUp [ref.meth]
unittest.rst:1657: WARNING: py:meth reference target not found: tearDown [ref.meth]
unittest.rst:1684: WARNING: py:meth reference target not found: defaultTestResult [ref.meth]
unittest.rst:1801: WARNING: py:meth reference target not found: TestSuite._removeTestAtIndex [ref.meth]
unittest.rst:1817: WARNING: py:meth reference target not found: TestSuite._removeTestAtIndex [ref.meth]
unittest.rst:1821: WARNING: py:class reference target not found: TestRunner [ref.class]
unittest.rst:1855: WARNING: py:class reference target not found: testCaseClass [ref.class]
unittest.rst:1858: WARNING: py:meth reference target not found: runTest [ref.meth]
unittest.rst:1908: WARNING: py:mod reference target not found: SampleTests [ref.mod]
unittest.rst:1908: WARNING: py:class reference target not found: SampleTestCase [ref.class]
unittest.rst:1908: WARNING: py:meth reference target not found: test_one [ref.meth]
unittest.rst:1908: WARNING: py:meth reference target not found: test_two [ref.meth]
unittest.rst:1908: WARNING: py:meth reference target not found: test_three [ref.meth]
unittest.rst:1908: WARNING: py:meth reference target not found: test_two [ref.meth]
unittest.rst:2058: WARNING: py:meth reference target not found: TestRunner.run [ref.meth]
unittest.rst:2145: WARNING: py:class reference target not found: TestRunner [ref.class]
unittest.rst:2150: WARNING: py:class reference target not found: TestRunner [ref.class]
unittest.rst:2471: WARNING: py:meth reference target not found: tearDownClass [ref.meth]
unittest.rst:2471: WARNING: py:meth reference target not found: setUpClass [ref.meth]

• Line 438: ~TestCase.__init__ are undocumented to-be-implemented function
• line 520: DocTestSuite is referring to doctest.DocTestSuite
• line 1025: exception is an undocumented attribute of the returned context manager.
• Line 1039: same as line 1026
• Line 1091: same. They areundocumented attributes of the returned context manager.
• Line 1440: the builtin-method set.difference() is document with frozenset.difference together with the reference of frozenset.difference, see here
• Line 1648, 1657, 1684: the method is inherented from TestCase
• Line 1801, 1817: Undocumented method of TestSuite, fine to leave it undocumented.
• Line 1821: The most common implementation of TestRunner is unittest.TestTestRunner. But here it is referring to all TestRunners that include certain methods. So suppressed as undocument class.
• Line 1855: testCaseClass is undocumented derived class from TestCase
• Line 1908: Sample tests. Not sure whether it is correct for this to be turned to inline-codeblock or suppressed links.
• Line 2058, 2145, 2150: Same as line 1821.
• Line 2471: They are to-be-implemented methods but documented in TestCase. So referring to TestCase.

• Issue: Fix all Sphinx reference warnings in the documentation #101100

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