[test] Overhaul `Undefined default export` test suite (#84235)
The test suite is now using fixture files instead of sandboxes, same principle as in #83398. I also removed two test cases that didn't test anything novel, compared to the other tests. They were originally moved from another test suite into this one in #66916.
This is done in preparation for adding build-time assertions.
> [!TIP]
> Best reviewed with hidden whitespace changes.
