Trim "mypy/tests/testX.py::XDataSuite::" from test items

We can rewrite test collection so that e.g. test-data/unit/check-abstract.test is deemed the test node, rather than mypy/test/testcheck.py::CheckTestSuite::check-abstract.test. This would improve the ergonomics of test invocation, so that one could simply invoke e.g.

pytest test-data/unit/check-abstract.test

Technically, we'd follow the recipe in Working with non-python tests. Each data suite would implement a pytest_collect_file. However, it would also mean we'd need a "registry" of all data suites, since the collection mechanism would need to "ask" all data suites whether they handle the file (in some cases, multiple data suites could each emit items for the same file & test -- see test_name_suffix).

The non-data-suite tests (e.g. those deriving from TestCase) would remain unaffected.

Start with the collection path for mypy/test/testcheck.py::CheckTestSuite::check-abstract.test and the data file test-data/unit/check-abstract.test. Read pytest's Working with non-python tests recipe, then trace how data suites and test_name_suffix currently identify items. Done means pytest can invoke the data file directly while non-data-suite TestCase tests remain unaffected.