Distribution of tests for unittest methods assertXXX()

While fixing the issue #71385 (pull request #96881) was confused about the distribution of tests for the unittest package. Concretely, I was working on the tests for assertAlmostEqual() that are placed in Lib/test/unittest/test_asserts.py. On the other side, the method assertLess() which tests as well relations of numbers is tested in test_case.py. For me it would be more clear to test them on the same position/file and move some tests accordingly. The idea to separate the assertXXX() tests into an own test_asserts.py is reasonable, so I would prefer to move assertLess() and its companions.