testsuite/init-module: error out init_module() on kmod failure

testsuite/init-module: error out init_module() on kmod failure

Currently our wrapper init_module() will happily return success whenever
libkmod fails. While such failures are unlikely, our wrapper should also
fail. In part so it doesn't mask a potentially deeper problem and in
part because the kmod API used, will set errno... Something a normal
syscall wouldn't do AFAICT.

v2:
 - remove the respective comment

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/346
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>