billy defines a common file system interface that is used in multiple go packages.
vfs.Handle implements billy.File mostly, only two methods needed to be added to
make it compliant.
An interface check is added as well.
This is a preliminary work for adding serve nfs command.
The tests are now run for the mount commands and for the plain VFS.
This makes the tests much easier to debug when running with a VFS than
through a mount.
