Consider testing from public API

[lread]

Originally raised by me as lread/rewrite-cljc-playground#25

rewrite-cljc uses import-vars to promote functions from internal namespaces to its public API.

But... tests are typically carried out against internal namespaces instead of the public API.

This means that tests won't catch import-vars errors.

[lread]

Another benefit is to folks who look at tests for usage, they will be less likely to use internal APIs.