selftests: mptcp: add explicit test case for remove/readd

[ Upstream commit b5e2fb83 ]

Delete and re-create a signal endpoint and ensure that the PM
actually deletes and re-create the subflow.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 1c2326fc ("selftests: mptcp: join: check re-adding init endp with != id")
Signed-off-by: Sasha Levin <sashal@kernel.org>