selftests: net: Unify code of busywait() and slowwait()

Bodies of busywait() and slowwait() functions are almost identical. Extract
the common code into a helper, loopy_wait, and convert busywait() and
slowwait() into trivial wrappers.

Moreover, the fact that slowwait() uses seconds for units is really not
intuitive, and the comment does not help much. Instead make the unit part
of the name of the argument to further clarify what units are expected.

Cc: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Benjamin Poirier <bpoirier@nvidia.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>