Fix test relying in wrong behavior of is_printable

[Jon Loeliger]

> Err,
> 
> I can rework and resubmit to remove the comment, but the test doesn't =
> fail:
> 
> > $ make check | grep 'MyBoardName.*-t s.*compatible'
> > fdtget-runtest.sh MyBoardName MyBoardFamilyName -t s label01.dts.fdtget.test.dtb / compatible:	PASS
> >
> 
> As of today's pull.

My build yields:

    $ make check 2>&1 | grep FAIL
    fdtget-runtest.sh 77 121 66 111 97 114 100 78 97 109 101 0 77 121 66 111 97 114 100 70 97 109 105 108 121 78 97 109 101 0 label01.dts.fdtget.test.dtb / compatible: FAIL Results differ from expected
    *                FAIL:	1

HTH,
jdl