Fix strange assertion failure, which hides real Block too big problem.

[TimSheard]

Pull request 3828, had a strange failure in the LEDGER rule in the Allegra era. It turns out that an underlying predicate failuure
RealChainPredicateFailure (BlockSizeTooLargeCHAIN 59587 54943) was setting the assertion failure off.
It appears that generating
Block traces ( Test.Cardano.Ledger.Shelley.Generator.Block(genBlockWithTxGen))
occasionally generates blocks that are slightly too big. So this is a failure of the test framework, not the system. We fix this by altering that function to discard, if the generated Block is too big.

This is also the cause of failures in PR 3848. Fixes #3851

Description

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated. If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages. New section is never added with the code changes. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[lehins]

Thank you for figuring this out, @TimSheard !