Alpha/DAL: add regressions tests for DAL rewards

The following discussion from !15614 (merged) should be addressed:

• @lrand started a discussion: (+1 comment)

  I don't see regressions in any tests that shows DAL rewards in the block receipts. This would be nice to have.

  This is not the scope of this MR though, so feel free to close the thread (with a follow-up issue if none exist, for instance)