Improve linera core tests (#4822)

## Motivation

The merging of PR #4805 led to a slight decrease in coverage for the
tests.

## Proposal

Add the tests for the certificate after the `transfer_to_account`,
`claim`, and `execute_operation`
that had been removed before.

## Test Plan

The CI.

## Release Plan

- Nothing to do / These changes follow the usual release cycle.

## Links

None.

Author: MathieuDutSik