Add unit tests for functions which do not have any test

[remyroy]

According to a recent coverage, we have a few functions that don't have any test. We don't need 100% coverage, but we should at least have a test that calls each important function at least once.

Here are a few that are missing tests:

• ethstaker_deposit/utils/validation.py: validate_bls_to_execution_change
• ethstaker_deposit/credentials.py: get_bls_to_execution_change
• ethstaker_deposit/credentials.py: get_bls_to_execution_change_dict
• ethstaker_deposit/credentials.py: save_exit_transaction
• ethstaker_deposit/credentials.py: deposit_datum_dict
• ethstaker_deposit/credentials.py: signed_deposit
• ethstaker_deposit/credentials.py: many parts of class Credential