Allow 'whitebox(...) { }' to be performed multiple times on the same Test Suite

## Use case

Do this in a convention plugin:

```
javaModuleTesting(test) {
   requires("junit") // for all my projects
}
```

And then this in a `build.gradle.kts` file on top

```
javaModuleTesting(test) {
   requires("something.else") // only for this project
}
```

Also:
- Repetitive `blackbox(...)` calls on the same Test Suite should not have any effect