Fix failing legacy chain tests

[teor2345]

Motivation

1. In some rare cases, a legacy chain proptest will fail, because it expects blocks that are always invalid.
2. The legacy chain tests check the blocks in the wrong order.

Solution

• skip test cases that are unexpectedly valid, in tests that require invalid blocks
• use the correct order for the block iterators in the tests
• add proptest seeds
• improve unclear documentation that might have caused these bugs

Review

@oxarbitrage can review this code, because he wrote the original legacy chain checks.

This fix is not urgent, unless CI starts failing with these bugs.

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour
• Tests for Errors

Follow Up Work

We might want to add a proptest enum with AlwaysValid, MaybeInvalid, and AlwaysInvalid variants.