s3-selftest: convert stream_depot vfstest driver into a subunit test

s3-selftest: convert stream_depot vfstest driver into a subunit test

This gives us our first automated coverage of the vfstest binary.

We don't use the simple smb.conf because we need to override all the
paths for this to work as non-root without a panic, so we use the s3dc
environment, which already loads this module.

Andrew Bartlett