The documentation (both for 1.6 at https://golang.org/cmd/go/ and for tip at https://tip.golang.org/cmd/go/) for go test says that

'Go test' recompiles each package along with any files with names matching the file pattern "*_test.go".

However the documentation for go build doesn't say anything about _test.go files.

I think the documentation for go build and go install should be updated to mention that _test.go files are ignored.

While most users probably do not read the documentation carefully it is still useful to have something to refer new users to. See the following StackOverflow question for background: http://stackoverflow.com/questions/36638149/undefined-variables-within-a-package-during-build