In analyze_outcomes.py, if a test case passes in reference_test but
not in driver_test, we log the key by key.format in python.
However, this causes error because of the grammar {} in python
string format. So removing {} to avoid KeyError for
sys.stderr.write((fmt + '\n').format(*args, **kwargs))
Signed-off-by: Yanray Wang <yanray.wang@arm.com>