testing: Move sysctl settings to sysctl.d and add some memory settings

Debian trixie doesn't provide a 99-sysctl.conf symlink in that directory
anymore.  The memory settings are also useful there as the default of
one changed and overbooking helps when forking a process with large
memory footprint (e.g. the IKE daemon).

testing/hosts/default/etc/sysctl.d/99-strongswan.conf

@@ -0,0 +1,16 @@
+# Enable packet forwarding for IPv4
+net.ipv4.ip_forward=1
+
+# Enable packet forwarding for IPv6
+#  Enabling this option disables Stateless Address Autoconfiguration
+#  based on Router Advertisements for this host
+net.ipv6.conf.all.forwarding=1
+
+# Enable coredump for suid binaries
+fs.suid_dumpable = 1
+
+# As we run with very little memory, use the old default to reduce overhead
+vm.max_map_count = 65530
+
+# Allow overcommitting, in particular for forks
+vm.overcommit_memory = 1