Make `TestCaseGenerator` use `UsvmSymbolicEngine` for entire class

[IlyaMuravjov]

Description

Makes it so UsvmSymbolicEngine is used for entire class at once, since it has its own budget management.

How to test

Should be tested when integration with USVM is completed.

Self-check list

• [x] I've set the proper labels for my PR (at least, for category and component).
• [x] PR title and description are clear and intelligible.
• [x] 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.
• [x] The documentation for the functionality I've been working on is up-to-date.