Alpha/DAL: add test for validity period of the accusations

The following discussion from !15832 (merged) should be addressed:

• @phink started a discussion: (+3 comments)

    let denunciation_period =

  Also, are we sure the denunciation period oldest element we need to store is
  max_slashing_period * blocks_per_cycle old? Even if we are in the middle of a cycle? Or do we consider the denunciation period to be max_slashing_period cycles before the current one?