testing: Add a test scenario for make-before-break reauth using a virtual IP

testing/tests/ikev2/reauth-mbb-virtual-ip/description.txt

@@ -0,0 +1,8 @@
+This scenario tests <b>make-before-break reauthentication</b> using overlapping
+IKE_SAs by setting the <i>make_before_break</i> strongswan.conf option for
+clients using an assigned virtual IP. The initiator <b>carol</b> reauthenticates
+the IKE_SA with host <b>moon</b> using <b>ikelifetime=10s</b>, but does not
+close the old IKE_SA before the replacement CHILD_SA using the same virtual IP
+is in place. A constant ping from <b>carol</b> to client <b>alice</b>
+hiding in the subnet behind <b>moon</b> tests if the CHILD_SA works during the
+whole procedure.