testing/networkmanager-l2tp: change libreswan to strongswan

libreswan currently cannot validate IKE, due to an assertion failure (see #11656 (closed)). A patch to the libreswan package is required to fix this, which is upstream of our package, but not yet implemented in 3.32. Libreswan additionally does not support 3des-sha1-modp1024, which is needed for l2tp VPNs created by devices like the Cisco Meraki. Strongswan has neither of these issues.