testsuite: s/return EXIT_SUCCESS/return 0/

testsuite: s/return EXIT_SUCCESS/return 0/

With all the EXIT_FAILURE instances done from the tests (only ones in
tools and the test runner remain), lets drop the EXIT_SUCCESS ones.

Functionally identical and arguably slightly neater.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/375
Signed-off-by: Lucas De Marchi <ldemarchi@kernel.org>