Refactor Class rendering in codegen (merge abstractions for regular and test classes)

[volivan239]

Description

Refactor class rendering

• for test classes
• for util classes
• support UtUtils class rendering in Kotlin
  by the way, fix known problems of Kotlin codegen that util methods are invisible

Fixes # (1255)
Fixes # (1256)

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Refactoring (typos and non-functional changes)

How Has This Been Tested?

Automated Testing

Standard utbot pipeline

Manual Scenario

Check base codegen scenarios in Java and Kotlin, requiring and not requiring UtUtils class.

Checklist (remove irrelevant options):

• The change followed the style guidelines of the UTBot project
• Self-review of the code is passed
• The change contains enough commentaries, particularly in hard-to-understand areas
• New documentation is provided or existed one is altered
• All tests pass locally with my changes