add ApprovableLedger unit test for self-transfer and remove it from its SMT

Description

Self-transfer, where the "from" and "to", is handled as a special case in ApprovableLedger tests, however this is not a restriction specified in its documentation.

The solution is to make the general unit test suite and SMT avoid this case, leaving its check to the specific contract tests, but add a unit test to check the common special-handling of self-transfer.

Related issue(s)

https://issues.serokell.io/issue/NBIT-33

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.