This commit increases the test coverage for the `pkg/datanode` package
from 58.1% to 66.9%.
The following changes were made:
- Added a new test file, `pkg/datanode/fromtar_test.go`, to test the
`FromTar` function.
- Added a new test file, `pkg/datanode/fs_test.go`, to test the `Stat`,
`Exists`, and `Walk` functions.
- Refactored the existing tests in `pkg/datanode/datanode_test.go` to
remove the monolithic test function and improve the organization of
the tests.
