selftests: mptcp: Restore packet capture option in join tests

The join self tests previously used the '-c' command line option to
enable creation of pcap files for the tests that run, but the change to
allow running a subset of the join tests made overlapping use of that
option.

Restore the capture functionality with '-c' and move the syncookie test
option to '-k'.

Fixes: 1002b89f ("selftests: mptcp: add command line arguments for mptcp_join.sh")
Acked-and-tested-by: Geliang Tang <geliangtang@gmail.com>
Co-developed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>