feat(rust/signed-doc): Signed Documents integration tests

[Mr-Leshiy]

Description

Updated the current catalyst-signed-doc integration tests. Provided more test cases and cleaned up the current implementation.

Related Issue(s)

Part of https://github.com/input-output-hk/catalyst-internal-docs/issues/266

Please confirm the following checks

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module

[github-actions]

✅ Test Report | ${\color{lightgreen}Pass: 466/466}$ | ${\color{red}Fail: 0/466}$ |

[apskhem]

LGTM