vfstests: move functional tests from mountlib and make them work with VFS

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.

vfs/vfstest/vfs.go

@@ -0,0 +1,20 @@
+package vfstest
+
+import (
+	"os"
+
+	"github.com/rclone/rclone/vfs"
+)
+
+// vfsOs is an implementation of Oser backed by the "vfs" package
+type vfsOs struct {
+	*vfs.VFS
+}
+
+// Stat
+func (v vfsOs) Stat(path string) (os.FileInfo, error) {
+	return v.VFS.Stat(path)
+}
+
+// Check interfaces
+var _ Oser = vfsOs{}