test(project): the #1851 no-requests test loads conda_vendor.py without the tomllib skip #1898

tests/project/test_conda_platform_pin_refresh_1851.py::FetchTests::test_without_requests_only_the_request_fails, added by #1885, fails on the Python 3.8 (KTLO), Python 3.9 (KTLO) and Python 3.10 (KTLO) unit legs with ModuleNotFoundError: No module named 'tomllib'. Seen on #1896's head 3a7a1cd51, whose base is main c9b482355.

The test loads util/conda_vendor.py itself, through spec.loader.exec_module, and that module imports tomllib at module scope, which needs 3.11. The file's other tests reach the script through the loader at :71-73, which raises SkipTest without tomllib. This one bypasses it.

KTLO legs read green even when tests fail (#1679), so this shows only in the annotations and the log.

Fix: give the test the same tomllib skip as the rest of the file, and add a check that fails if a test in the file loads the script without that guard.