iotests/297: Create main() function

iotests/297: Create main() function

Instead of running "run_linters" directly, create a main() function that
will be responsible for environment setup, leaving run_linters()
responsible only for execution of the linters.

(That environment setup will be moved over in forthcoming commits.)

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-id: 20211019144918.3159078-5-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>