Sometimes an external test package may need privileged access to the internals of the package under test, if for example a white-box test must live in a separate package to avoid an import cycle. In such cases, we use a trick: we add declarations to an in-package _test.go file to expose the necessary internals to the external test. This file thus offers the test a “back door” to the package. If the source file exists only for this purpose and contains no tests itself, it is often called export_test.go.
