selftests: netdevsim: add carrier state consistency test

selftests: netdevsim: add carrier state consistency test

This commit adds a test case for netdevsim carrier state consistency.
Specifically, the added test verifies the carrier state during the
following operations:

1. Unlink two netdevsims
2. ifdown one netdevsim, then ifup again
3. Link the netdevsims again
4. ifdown one netdevsim, then ifup again

These steps verifies that the carrier is UP iff two netdevsims are
linked and ifuped.

Signed-off-by: Yohei Kojima <yk@y-koj.net>
Link: https://patch.msgid.link/481e2729e53b6074ebfc0ad85764d8feb244de8c.1767624906.git.yk@y-koj.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>