Add a test for producing one shard block per master block

[SmaGMan]

Scope

• Base the new test on test_collation_process_on_dump in collation_tests.rs.
• Create an anchor dump with a single external message.
• Given the current logic, after a shard block is produced and there are no unprocessed messages, the next master block should be forced, resulting in exactly one shard block per master block.
• Assert: a shard block 1 is produced, then a master block 1 is produced, then the next shard block 2 is produced correctly.