improvement: add proper_test_support_location/1

[ibarakaiev]

Contributor checklist

• Bug fixes include regression tests
• Features include unit/acceptance tests

This PR adds proper_test_support_location/1, which returns idiomatic test support location such as MyApp.DataCase -> test/support/data_case.ex. It also inlines the tests as doctests for readability.

[zachdaniel]

We should make a patcher that adds test/support to your libc directories, and then call it whenever you add a file into test/support.

[zachdaniel]

🚀 Thank you for your contribution! 🚀