tests: use NO_CHECK_EOF for fragments used to test importing via extension test-extension.t is excluded in test-check-module-imports.t, because import-checker.py reports that some python code fragments in it does not import modules in recommended style. These code fragments are used to test importing modules relatively and absolutely via extension (e.g. issue4029, issue5208, and so on). Test specific package hierarchy becomes complicated for this purpose, and it seems reasonable to avoid checking these code fragments with import-checker.py. But on the other hand, other code fragments in test-extension.t should be checked by import-checker.py. Therefore, this patch uses NO_CHECK_EOF limit mark only for python code fragments, which are used to test importing via extension in test-extension.t. NO_CHECK_EOF limit mark tells import-checker.py that this code fragment should be ignored, via testparseutil.py.