builder, sysprep: initialise the random generator

virt-builder and virt-sysprep may make use of
Common_utils.string_random8 (which uses Random.int) for constructing
temporary paths; not initialising the random generator means that every
invocation will reuse the same name used previously (!).
Thus just call Random.self_init, just like virt-sparsify already does.

Expand the test-virt-sysprep-script.sh test to ensure that virt-sysprep
is not affected again by this issue.