Add passwordless test helpers

[rickychilcott]

This is a PR to add test helpers for test unit and rspec. I tried this locally on a project I'm using and it works ok, but I'm only using rspec. Someone who has a good example test unit project should try this or some tests can be integrated into a passwordless example app.

This takes a lot of code and ideas from #71 so no credit is taken here, just trying to pull together to get to this as part of a 1.0 release (#89).

Close #71 and address #29

[mikker]

Thank you for this @rickychilcott!

They'll have to be modified a bit for 1.0 but that doesn't mean they aren't a great addition. A few remarks and suggestions, otherwise looks great!

[rickychilcott]

Thanks @mikker. I made the requested changes.

I was modeling some of the syntax off of https://github.com/thoughtbot/clearance#controller-test-helpers but I looked too quickly and didn't realize that they effectively shim the current_user if you just did a sign_in without doing sign_in_as(user), so I dropped my weird overloading of the passwordless_sign_in/passwordless_sign_in(user) form.

Let me know if you'd like any other changes.

[mikker]

💙💚💛💜❤️