Fixed mocking fields in another classes in concrete execution

[Damtev]

Description

Allows to mock fields of another class in the concrete execution.

Fixes #2105.

How to test

Automated tests

org.utbot.examples.mock.fields.ClassUsingClassWithRandomFieldTest.

Manual tests

The test mentioned in the issue.

Self-check list

• I've set the proper labels for my PR (at least, for category and component).
• PR title and description are clear and intelligible.
• I've added enough comments to my code, particularly in hard-to-understand areas.
• The functionality I've repaired, changed or added is covered with automated tests.
• Manual tests have been provided optionally.
• The documentation for the functionality I've been working on is up-to-date.