testsuite: remove #define pr_fmt

testsuite: remove #define pr_fmt

The kernel headers already provide a default which is sufficient for out
needs.

Inspired by a few new test modules which set it, although never call
pr_warn() or friends.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/333
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>