tests/frontend/order.py: Adding tests ensuring stable element processing order

For each sample project and expected result order, this test ensures that:

  * bst show --deps plan: Always shows the expected build order.

  * bst source fetch: Always executes in the expected build order.

  * bst build: Always builds in the expected order (disabled).

The build part of the test is currently disabled as the scheduler
seems to be messing with the order even in a `--builders 1` scenario.