selftests: mptcp: gives slow test-case more time

On slow or busy VM, some test-cases still fail because the
data transfer completes before the endpoint manipulation
actually took effect.

Address the issue by artificially increasing the runtime for
the relevant test-cases.

Fixes: ef360019 ("selftests: mptcp: signal addresses testcases")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/309


Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>