selftests: mptcp: run mptcp_sockopt from a new netns

Not running it from a new netns causes issues if some MPTCP settings are
modified, e.g. if MPTCP is disabled from the sysctl knob, if multiple
addresses are available and added to the MPTCP path-manager, etc.

In these cases, the created connection will not behave as expected, e.g.
unable to create an MPTCP socket, more than one subflow is seen, etc.

A new "sandbox" net namespace is now created and used to run
mptcp_sockopt from this controlled environment.

Fixes: ce997912 ("selftests: mptcp: add mptcp getsockopt test cases")
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>