pdtest: use less expensive Python routine to get text differences

pdtest: use less expensive Python routine to get text differences

Python ships with a difflib(3) module which is used in the pdtest(1)
utility. The Differ.compare() routine is rather expensive, especially
when the set of input text becomes "large" (a few thousand lines). Use
the less expensive unified_diff() routine instead which does not suffer
from that cost. From the sigrok-test perspective the resulting data is
as usable.