Use VirtualStorageProvider::new_overlay(test_data()) in tests

[arrayka]

Update the unit tests so that any hardcoded test_data in string paths and workspace_root usages are replaced with VirtualStorageProvider::new_overlay(test_data_root()).

Here is an example of expected pattern:

        let storage_provider = VirtualStorageProvider::new_overlay(test_data_root());
        let dataset_file = "/sift/siftsmall_learn.bin";
        let mut file = storage_provider.open_reader(dataset_file).unwrap();

Ensure that all unit tests still pass.

Original comment:

Something we should also do (maybe in another PR) is to replace all uses of test_data with the file system resolution in diskann-utils. I think a few fell through the cracks when test data was consolidated.

Originally posted by @hildebrandmw in #700