twister: add options to shuffle tests across subsets

Add option to shuffle tests (randomly) before they are split into subsets.
This allow to fairly distribute tests across subsets,
so that long tests are not crowded in a single group.

This also allows to detect unwanted dependencies in test execution order.

Also, added option to provide custom seed to random generator
used to shuffle tests.
This will allow to reproduce certain test order if needed.
Used seed is printed at console.

Signed-off-by: Piotr Kosycarz <piotr.kosycarz@nordicsemi.no>