test: use new test helper to make test more robust

Replace timed wait by wait for eos. Add a define for the tick-count.
song: back to seek in PAUSED

Something changed in gstreamer to break seek in READY again. The pipeline plays
from the right pos, but reports position starting from 0 instead of seek-pos.
Now we waste some CPU for preroll and then flushing with the actual seek :/

Add a test for the pos queries after seek.