HPCC-12343 Missing content of diff result when a test case output doesn't
match to key file.

Fix Unicode string handling problem occured in Python 2.6.6. difflib.

Signed-off-by: Attila Vamos <attila.vamos@gmail.com>
Merge pull request #6497 from AttilaVamos/HPCC-12343-fix

HPCC-12343 Missing content of diff result when a test case output doesn't  match to key file

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>