Free the memory allocated to store the test FDT upon test completion to
avoid leaking the memory. We don't bother cleaning up on test failure
since the code is broken in that case and should be fixed, in which case
the leak would also go away.

Reported-by: Tom Rini <tom.rini@gmail.com>
Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>