double-merge malfeasance proof

[poszu]

Motivation

Closes #6341

Description

A malfeasance proof for when more than 1 merged ATX is published in a single epoch by a single equivocation set. Covers only the simple case when no checkpoint is involved. Support for checkpoint will come in following PRs

Test Plan

Added tests, skipped the test for when the first merged ATX is checkpointed - this should be IMO a syntactically invalid ATX.

TODO

• Explain motivation or link existing issue(s)
• Test changes and document test plan
• Update documentation as needed
• Update changelog as needed

[codecov]

Codecov Report

Attention: Patch coverage is 82.32044% with 32 lines in your changes missing coverage. Please review.

Project coverage is 81.8%. Comparing base (87893ee) to head (893b690).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
activation/wire/malfeasance_double_merge.go	83.0%	13 Missing and 13 partials ⚠️
activation/handler_v2.go	33.3%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #6339    +/-   ##
========================================
  Coverage     81.8%   81.8%            
========================================
  Files          312     313     +1     
  Lines        34606   34775   +169     
========================================
+ Hits         28314   28459   +145     
- Misses        4461    4466     +5     
- Partials      1831    1850    +19     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.