TestKit should expose some of its configurations and conventions for use by other plugins

**Is your feature request related to a problem? Please describe.**
I'm developing a Gradle plugin that uses test kit. It's impossible to get some things, like the `includedBuilds` the user specified. The reason being that I'm creating multiple `functionalTest` tasks with modified arguments and such.

There's also no way programmatic way to get names for `functionalTest` source set, tasks, etc

**Describe the solution you'd like**
Expose some of these properties publicly, so other plugins can depend on them

**Describe alternatives you've considered**
I'm hardcoding `functionalTest` names for now. I'm also copying `jvmArgumentsProviders`, but having to filter for `GradleJvmCommandLineArgumentProvider` because it breaks configuration cache.

Start with the TestKit configuration around the functionalTest source set and tasks, then trace includedBuilds and jvmArgumentsProviders, including GradleJvmCommandLineArgumentProvider. Determine which names and properties can be exposed without breaking configuration-cache behavior; done when another plugin can use them without hardcoded functionalTest names or copied providers.

Written by the indexing model from the issue text.