selftests: silence test output by default

Some of the networking tests are very noisy and make it impossible to
see if we actually passed the tests as they run.  Default to suppressing
the output from any tests run in order to make it easier to track what
failed.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>