move TestHelpers.OAs -> Test.OAs

[rfourquet]

This is motivated by the fact that I didn't find a way to use the OAs module in Random tests (in #24874, where I had to duplicate temporarily OAs), and that after all this OAs module could very well be useful beyond base/stdlib (i.e. in external packages).
I'm not sure why the module name is not OffsetArrays - maybe to not be confused with the "real" module which is in a package? (cc @timholy).

[KristofferC]

I don't think this should be in the Test module. It is just a utility thing to help test some Base functionality. If people want to test their stuff with OffsetArrays in their packages, they can just use the OffsetArrays package?

[StefanKarpinski]

Agree, this seems strange. Keep in mind that Test is a formal interface that we support. Having random (forgive the pun) stuff in it just for Base's own testing is not ideal.

[rfourquet]

Ok, I agree, and I also remember now that a package can depend on another one only for tests, which makes this dependency optional for using the package (so depending on OffsetArrays for testing is not a big deal).

[rfourquet]

For reference, another (better) fix at #25582.