by rafrombrc:

One feature that I have wished to have is the ability to export names from the *_test.go
files in one package such that they can only be used by *_test.go files in other
packages. This is not possible, but it occurred to me recently that this can be
approximated by the use of compiler build tags. In fact, if the test tool automatically
injected a standard build tag (e.g. "testrun"), then it would be trivial to
add files to a package containing code that would only be exported when tests were being
run. (The files couldn't be named "*_test.go", of course, but the effect would
be the same.)