Move shared test code to testkit module

### Use case

Modules should not depend on tests of the other modules.
The current setup with `testImplementation(project(":src:core", "testClasses"))` dependencies models what we had with Ant, however, that pattern is hard to maintain.

### Possible solution

A set of shared classes should be identified and moved to `/src/testkit` or a similar module.

### Possible workarounds

_No response_

### JMeter Version

5.5

### Java Version

_No response_

### OS Version

_No response_