cmd/go: mention that _test.go files are ignored when building

Fixes #15315

Change-Id: I8fea31507a5f83df8a86fb067f1b11d90133dc09
Reviewed-on: https://go-review.googlesource.com/22180Reviewed-by: Chris Broadfoot <cbro@golang.org>

src/cmd/go/alldocs.go

@@ -69,6 +69,8 @@ When compiling multiple packages or a single non-main package,
 build compiles the packages but discards the resulting object,
 serving only as a check that the packages can be built.
 
+When compiling packages, build ignores files that end in '_test.go'.
+
 The -o flag, only allowed when compiling a single package,
 forces build to write the resulting executable or object
 to the named output file, instead of the default behavior described

src/cmd/go/build.go

@@ -48,6 +48,8 @@ When compiling multiple packages or a single non-main package,
 build compiles the packages but discards the resulting object,
 serving only as a check that the packages can be built.
 
+When compiling packages, build ignores files that end in '_test.go'.
+
 The -o flag, only allowed when compiling a single package,
 forces build to write the resulting executable or object
 to the named output file, instead of the default behavior described