Currently, adding new int() and long() tests that are common to both requires duplicating the tests in both test_int and test_long, which makes the tests harder to review and maintain and keep in synch. Providing a simple way to share tests between test_int and test_long will simplify adding new tests for issues like issue 16784, issue 16761, and issue 16772. This issue isn't applicable to 3.x because in 3.x test_int and test_long both test int(), so it doesn't make sense to repeat tests in both.
Attaching patch. The tests moved to SharedTestCaseMixin in this patch are tests that were very recently added (as part of issue 16045) and that are planned on being added to test_long as part of issue 16784.
After this issue, the ordering of new test_int test cases in 3.x should preferably match the ordering in 2.7 for easier back-porting when the tests are new in both. For example, test_no_args() and test_keyword_args() could go at the top of 3.x's IntTestCases.