Context

There was already a regression test covering the inbox_size for sc rollups - this MR adds a test that covers:

the value of current_messages_hash when no messages are sent to the rollup
that the messages can be retrieved from the block, and used to reconstruct the current_messages_hash

Manually testing the MR

dune exec tezt/tests/main.exe -- --file sc_rollup.ml

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Provide automatic testing (see the testing guide).
For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
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Edited Apr 22, 2022 by Emma Turner