mypy

ce0703ac - move testcase to proper file, change its name and use more concise name for variable (#8112)

move testcase to proper file, change its name and use more concise name for variable (#8112) Followed the discussion in https://github.com/python/mypy/pull/8107#issue-350416074, refine the code merged in #7662 by: - move testcase from `pythoneval.test` to `check-namedtuple.test` and rename it from `testNamedTupleAtRunTime` to `testAssignNamedTupleAsAttribute` - change `s.lvalues[0]` to `lvalue` to make it more concise