of: unittest: Remove error printing on OOM

There is no need to print a backtrace or other error message if
kzalloc(), kmemdup(), or devm_kzalloc() fails, as the memory allocation
core already takes care of that.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>