testing: mention the “_test” package idiom

[bcmills]

https://tip.golang.org/cmd/go/#hdr-Test_packages says:

Test files that declare a package with the suffix "_test" will be compiled as a separate package, and then linked and run with the main test binary.

However, https://tip.golang.org/pkg/testing/ omits that detail (emphasis mine):

To write a new test suite, create a file whose name ends _test.go that contains the TestXxx functions as described here. Put the file in the same package as the one being tested.

Tests in a separate _test package provide more realistic examples (and are better for catching awkward package names), so it's important that users be aware of that option. The testing package is an important entry point for users who are new to Go: its documentation should mention both package options or neither.

[rillig]

After programming in Go for three years, I learned about this feature in this Stack Overflow answer. If the one sentence mentioning white-box and black-box testing had been in the reference documentation or the Effective Go document, it would have helped me quite a bit.

[gopherbot]

Change https://golang.org/cl/145279 mentions this issue: testing: mention the "_test" package idiom

[hearot]

At least, now it is a bit mentioned. I may expand its documentation.