tests: more flexible handling mender-artifact rootfs size
tests: more flexible handling mender-artifact rootfs size

MENDER_CALC_ROOTFS_SIZE is the size of rootfs partition. It may happen that the
actual contents of rootfs are significantly smaller than the desired partition
size. Assume that the compressed contents will not be less than 30% of rootfs
partition size.

Changelog: none

Signed-off-by: Maciej Borzecki <f609d9f713abcd5f8f0e51f08dc151638a7681e7@rndity.com>

bboozzoo/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender