Add support for running content updates against an existing migration
test dataset without recreating the filesystem structure.
Also make ACL/xattr updates non-fatal on filesystems that do not
support those operations.
Add optional interval-based random content updates to the cdssync
migration test dataset generator and document the new behavior.
This allows the dataset to be created once and then updated either
continuously or every N seconds while preserving the intended
special-case file structure.
Add the cdssync migration test dataset manifest, generator script,
workspace instructions, and gitignore.
This sets the default workflow to:
- generate the dataset locally
- copy it to the test machine with metadata preserved
- verify the copied data before migration testing
