ci: Extend FAH rootfs for installed tests

These at the moment aren't in a container, and may need space. In the future
overlay2 will help here, we can more easily extend the rootfs.

Closes: #840
Approved by: jlebon

tests/installed/fah-prep.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+set -xeuo pipefail
+# If we're using devmapper, expand the root
+if lvm lvs atomicos/docker-pool &>/dev/null; then
+    systemctl stop docker
+    lvm lvremove -f atomicos/docker-pool
+fi
+lvm lvextend -r -l +100%FREE atomicos/root