selftests: mptcp: add set_flags tests in pm_netlink.sh

This patch added the setting flags test cases, using both addr-based and
id-based lookups for the setting address.

The output looks like this:

 set flags (backup)                                 [ OK ]
           (nobackup)                               [ OK ]
           (fullmesh)                               [ OK ]
           (nofullmesh)                             [ OK ]
           (backup,fullmesh)                        [ OK ]

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>