Improve tests for CreateProposalBlock #3156
Description
Follow up to #2806, see the tests in
Line 202 in 0726329
Improvements to be made include:
- use real evidence for different blockIDs instead of introducing this new mock evidence
- ensure the Commit and Header are at maximum size
- confirm this would have caught Method of deciding amino overhead of tx in ReapMaxBytesMaxGas is wrong #2789
- tests with more variability in the block structure