Keep thisInstance modifications in controller-specific tests

[IlyaMuravjov]

Description

In controller-specific integration tests thisInstance modifications were lost (i.e. saving data to database, invoking controller methods before method under test), this PR restores such modifications.

How to test

Manual tests

Generate tests on the example from #2562, there should be tests that save data to database directly or use controller methods to do that.

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.