testing/soju: create /var/lib/soju

Without this, soju fails to start with:

    daemon.info supervise-daemon[5733]: Supervisor command line: supervise-daemon soju --start --chdir /var/lib/soju --stderr /var/log/soju.log --user soju:soju /usr/bin/soju -- -config /etc/soju/config
    daemon.err /etc/init.d/soju[5736]: supervise-daemon: chdir `/var/lib/soju': No such file or directory
8 jobs for !26359 with soju-var-dir in 5 minutes and 41 seconds (queued for 4 seconds)
detached
Status Job ID Name Coverage
  Lint
passed #510756
docker-alpine x86_64
lint

00:00:20

 
  Build
passed #510761
docker-alpine ci-build aarch64
build-aarch64

00:04:27

passed #510763
docker-alpine ci-build armhf
build-armhf

00:04:25

passed #510762
docker-alpine ci-build armv7
build-armv7

00:04:32

passed #510760
docker-alpine ci-build ppc64le
build-ppc64le

00:05:19

passed #510759
docker-alpine ci-build s390x
build-s390x

00:02:12

passed #510758
docker-alpine ci-build x86
build-x86

00:03:01

passed #510757
docker-alpine ci-build x86_64
build-x86_64

00:02:10