test: call hexdump tests via `ut lib`

The unit tests in test/lib/hexdump.c are not related to the device tree.
So they should be executed via `ut lib` and not via `ut dm`.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
1 file changed