Draft: Tezt/adaptive_issuance: no longer hard-code expected slashed amounts

What

In the Adaptive Issuance test Staking - test staking with delegate and staker in a simple scenario, actually compute expected slashed amounts instead of hard-coding them.

Why

This will avoid the need for manual updates each time we modify the accounts' initial amounts, or rewards, or constants such as blocks per cycle, etc.

Manually testing the MR

dune exec tezt/tests/main.exe -- -m "Staking - test staking with delegate and staker in a simple scenario"