Increase access level for MultiEnvTestEngine helpers

[colan-dremio]

Several existing helpers are currently package private and would be helpful to expose for re-use in other packages.

[CLAassistant]

All committers have signed the CLA.

[colan-dremio]

At present, this represents a set of classes/methods that I am explicitly planning to use. It leaves a few classes in a state that is partially public and partially package private. If there is interest, I'm happy to make more items public for consistency and reuse elsewhere too.

[snazy]

The changed classes are all in the internal package of the test extension, those were not intended to be used externally.

Can you elaborate which functionality is missing and maybe have a PR that add the potentially missing functionality to the JUnit extension rather than relying on internal behavior of the test extension?

[colan-dremio]

The primary use case was to support more than one MultiEnvTestExtension. I've created #7816 as a more robust solution.