switch to a simpler DTMF signal

we don't really know why the DTMF detection is so unreliable, but the sound
quality of the tests *does* differ - but we don't want to program phones,
we just want to check if there is audio.

So possibly long signals of less letters will do
Merge pull request #92 from os-autoinst/coolo_159D

switch to a simpler DTMF signal