[3.8] Slightly improve plistlib test coverage. (GH-17025) (GH-17028)

* Add missing test class (mistake in GH-4455)

* Increase coverage with 4 more test cases

* Rename neg_uid to huge_uid in test_modified_uid_huge

* Replace test_main() with unittest.main()

* Update plistlib docs.
(cherry picked from commit d0d9f7cfa36bafa4e1d9e73eb08835180d376df1)

Co-authored-by: Jon Janzen <jjjonjanzen@gmail.com>
2 files changed