selftests: mptcp: do complete cleanup at exit

After commit 05be5e2 ("selftests: mptcp: add disconnect tests")
the mptcp selftests leave behind a couple of tmp files after
each run. run_tests_disconnect() misnames a few fariables used to
track them. Address the issue setting the appropriate global variables

Fixes: 05be5e2 ("selftests: mptcp: add disconnect tests")
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

tools/testing/selftests/net/mptcp/mptcp_connect.sh

@@ -782,8 +782,8 @@ run_tests_disconnect()
 	run_tests_lo "$ns1" "$ns1" dead:beef:1::1 1 "-I 3 -i $old_cin"
 
 	# restore previous status
-	cout=$old_cout
-	cout_disconnect="$cout".disconnect
+	sin=$old_sin
+	sin_disconnect="$cout".disconnect
 	cin=$old_cin
 	cin_disconnect="$cin".disconnect
 	connect_per_transfer=1