Publish adapter-test package

[hparra]

Description 📓

I thought adapter-test was an actual package but it doesn't seem so. It would be nice to have formal testing support for custom adapters.

I'll copy/paste code for now. Thanks for this lib!

How to reproduce ☕️

# expected something like the following to work
npm install --save-dev @next-auth/test-adapter

Contributing 🙌🏽

No, I am afraid I cannot help regarding this

[spaceandrew]

Second this! Since it's listed in packages, definitely thought it was accessible.