[TM-78] Improve integrational testing interface and examples

Description

This MR does two things:

1. It improves interface for integrational testing.
2. It makes existing integrational test (stringCaller) more advanced to demonstrate some features.
3. It adds another integrational test which better demonstrates random blockchain state generation.

Commit messages provide more info.

Related issue(s)

https://issues.serokell.io/issue/TM-78

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests

  • 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:
    • README
    • Haddock
    • docs/

Stylistic guide (mandatory)

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