Implement new method Resolve of kopia's fs.Symlink

We have a mock type in kanister that implements kopia's `fs.Symlink`
interface. Since this interface was recently modified to have a new
method the mock type that we have in Kanister started to not implement
that.
This commit makes sure that we add a dummy implementation of the
type.

pkg/virtualfs/symlink.go

@@ -30,3 +30,7 @@ var _ fs.Symlink = (*inmemorySymlink)(nil)
 func (imsl *inmemorySymlink) Readlink(ctx context.Context) (string, error) {
 	panic("Symlinks not supported")
 }
+
+func (imsl *inmemorySymlink) Resolve(ctx context.Context) (fs.Entry, error) {
+	panic("Resolve for mock implementation is not supported")
+}