tests/comparators: dtb: compatibility with version 1.4.5

Used to be:

% fdtdump --version
Version: DTC 1.4.2-dirty
%

Now it's instead:

% fdtdump --version

**** fdtdump is a low-level debugging tool, not meant for general use.
**** If you want to decompile a dtb, you probably want
****     dtc -I dtb -O dts <filename>

Version: DTC 1.4.5
%

So the tests broke… (even worse, the whole testsuite broke as pytest
couldn't compute the list of tests to execute…)

Signed-off-by: Mattia Rizzolo <mattia@debian.org>