selftests/bpf: Test setting and creating bpf qdisc as default qdisc

First, test that bpf qdisc can be set as default qdisc. Then, attach
an mq qdisc to see if bpf qdisc can be successfully created and grafted.

The test is a sequential test as net.core.default_qdisc is global.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>