progress: use 'encoding.trim' to trim output line correctly Before this patch, 'progress' extension trims output line by directly slicing byte sequence, but it may split at intermediate multi-byte sequence. This patch uses 'encoding.trim' to trim output line correctly, even if it contains multi-byte characters. "rm -f loop.pyc" before changing "loop.py" in "test-progress.t" ensures that re-compilation of "loop.py", even if "loop.py" and "loop.pyc" have same timestamp in seconds.