Speed up some PoS tests

[brentstone]

Describe your changes

Speed up test_purge_validator_information and test_multiple_misbehaviors, as they previously take too long to run in CI.

Indicate on which release or other PRs this topic is based on

v0.28.1

Checklist before merging to draft

• [x] I have added a changelog
• [x] Git history is in acceptable state

[Fraccaman]

can we also make them run with make test-pos-sm ?