testing: add example to package doc

The package doc for the testing package doesn't have a simple
example demonstrating how to write a test with an expectation. The doc
has simple examples for benchmarks, examples, and skipping, and it would be
useful for people new to writing tests in Go.

Also moved the skip example further down because it references tests and
benchmarks but benchmarks haven't been discussed in detail until the
next section. Skip is also a less used feature and it seems misplaced to
sit so high up in the package documentation. As an example, Skip is used
570 times the Go code repository which is significantly less than Error
and Fatal that are used 23,303 times.

Also changed 'sample' to 'simple' in other places in the package documentation
to keep the language used consistent when describing the small examples.

Fixes #27839

Change-Id: Ie01a3751986ee61adf2a2f2eda59cc182342baa7
GitHub-Last-Rev: 7357bfdcd29ed1dc1719c9436b5d5420020610ee
GitHub-Pull-Request: golang/go#27840
Reviewed-on: https://go-review.googlesource.com/c/137175
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/testing/testing.go

@@ -17,13 +17,13 @@
 // package builds but will be included when the ``go test'' command is run.
 // For more detail, run ``go help test'' and ``go help testflag''.
 //
-// Tests and benchmarks may be skipped if not applicable with a call to
-// the Skip method of *T and *B:
-//     func TestTimeConsuming(t *testing.T) {
-//         if testing.Short() {
-//             t.Skip("skipping test in short mode.")
+// A simple test function looks like this:
+//
+//     func TestAbs(t *testing.T) {
+//         got := Abs(-1)
+//         if got != 1 {
+//             t.Errorf("Abs(-1) = %d; want 1", got)
 //         }
-//         ...
 //     }
 //
 // Benchmarks
@@ -132,6 +132,18 @@
 // example function, at least one other function, type, variable, or constant
 // declaration, and no test or benchmark functions.
 //
+// Skipping
+//
+// Tests or benchmarks may be skipped at run time with a call to
+// the Skip method of *T or *B:
+//
+//     func TestTimeConsuming(t *testing.T) {
+//         if testing.Short() {
+//             t.Skip("skipping test in short mode.")
+//         }
+//         ...
+//     }
+//
 // Subtests and Sub-benchmarks
 //
 // The Run methods of T and B allow defining subtests and sub-benchmarks,