Take benefit from squashfs-tools 4.3 for faster test builds

Initial discussion and benchmarks: https://mailman.boum.org/pipermail/tails-dev/2015-July/009241.html

squashfs-tools 4.3 allows setting the gzip compression level. This means faster test builds.

Once our Vagrant basebox is based on Stretch, we can modify the gzipcomp build option to use a lower gzip compression level.

Related issues

• Related to #16177 (closed)

Original created by @romeopapa on 9788 (Redmine)