iotests: Set LC_ALL=C for sort

iotests: Set LC_ALL=C for sort

Otherwise the result is basically unpredictable.

(Note that the precise environment variable to control sorting order is
LC_COLLATE, but LC_ALL overrides LC_COLLATE, and we do not want the
sorting order to be messed up if LC_ALL is set in the environment.)

Reported-by: John Snow <[email protected]>
Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Alex Bennée <[email protected]>
Message-Id: <20200710163253[email protected]>