testing: structure package documentation

Makes finding the relevant information much easier.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/8353045

src/pkg/testing/testing.go

@@ -18,6 +18,8 @@
 //         ...
 //     }
 //
+// Benchmarks
+//
 // Functions of the form
 //     func BenchmarkXxx(*testing.B)
 // are considered benchmarks, and are executed by the "go test" command when
@@ -49,6 +51,8 @@
 //         }
 //     }
 //
+// Examples
+//
 // The package also runs and verifies example code. Example functions may
 // include a concluding line comment that begins with "Output:" and is compared with
 // the standard output of the function when the tests are run. (The comparison