testsuite: busy_sim: Use xoshiro128++ when enabled

Taking random numbers from hardware may be time consuming thus
it was deferred to a work queue. On the other hand, taking them
from software algorithm is fast. Use xoshiro128++ when enabled,
instead of real random numbers from hardware RNG generator. They
are use for random intervals in the test so no security concerns
here.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>