Generate extensions code when it's required

[viktoriia-fomina]

Description

When tests are generated with VSharp.Runner, tests extensions are always included into generated tests.
When we generate tests without special option --single-file they are generated into Extensions directory. When we use special option --single-file extensions are generated into file with tests.

Expected

Extensions are generated only when they are required for tests.

To reproduce

VSharp console runner with special option --single-file:

dotnet /usr/src/vsharp_runner/VSharp.Runner.dll --all-public-methods /usr/src/project/bin/Debug/net6.0/project.dll --render-tests --single-file