Reduce the number of rounds in DecoderCrashTest

Currently this one single test accounts for 12-20% of the total
runtime of running the unit test suite.
Use & 0xff instead of WELS_CLIP3(, 0, 255)

This speeds up the whole EncoderInterfaceTest to about double
the speed from before.
Merge pull request #1533 from mstorsjo/speedup-tests

Speed up some slow tests in the unit test suite