testing: add T.Context method

From the doc comment:

Context returns the context for the current test or benchmark.
The context is cancelled when the test or benchmark finishes.
A goroutine started during a test or benchmark can wait for the
context's Done channel to become readable as a signal that the
test or benchmark is over, so that the goroutine can exit.

Fixes #16221.
Fixes #17552.

Change-Id: I657df946be2c90048cc74615436c77c7d9d1226c
Reviewed-on: https://go-review.googlesource.com/31724
Reviewed-by: Rob Pike <r@golang.org>

src/testing/sub_test.go

@@ -6,6 +6,7 @@ package testing
 
 import (
 	"bytes"
+	"context"
 	"regexp"
 	"strings"
 	"sync/atomic"
@@ -277,28 +278,33 @@ func TestTRun(t *T) {
 		ok:     true,
 		maxPar: 4,
 		f: func(t *T) {
-			t.Parallel()
-			for i := 0; i < 12; i++ {
-				t.Run("a", func(t *T) {
-					t.Parallel()
-					time.Sleep(time.Nanosecond)
-					for i := 0; i < 12; i++ {
-						t.Run("b", func(t *T) {
-							time.Sleep(time.Nanosecond)
-							for i := 0; i < 12; i++ {
-								t.Run("c", func(t *T) {
-									t.Parallel()
-									time.Sleep(time.Nanosecond)
-									t.Run("d1", func(t *T) {})
-									t.Run("d2", func(t *T) {})
-									t.Run("d3", func(t *T) {})
-									t.Run("d4", func(t *T) {})
-								})
-							}
-						})
-					}
-				})
-			}
+			// t.Parallel doesn't work in the pseudo-T we start with:
+			// it leaks a goroutine.
+			// Call t.Run to get a real one.
+			t.Run("X", func(t *T) {
+				t.Parallel()
+				for i := 0; i < 12; i++ {
+					t.Run("a", func(t *T) {
+						t.Parallel()
+						time.Sleep(time.Nanosecond)
+						for i := 0; i < 12; i++ {
+							t.Run("b", func(t *T) {
+								time.Sleep(time.Nanosecond)
+								for i := 0; i < 12; i++ {
+									t.Run("c", func(t *T) {
+										t.Parallel()
+										time.Sleep(time.Nanosecond)
+										t.Run("d1", func(t *T) {})
+										t.Run("d2", func(t *T) {})
+										t.Run("d3", func(t *T) {})
+										t.Run("d4", func(t *T) {})
+									})
+								}
+							})
+						}
+					})
+				}
+			})
 		},
 	}, {
 		desc:   "skip output",
@@ -341,6 +347,7 @@ func TestTRun(t *T) {
 			},
 			context: ctx,
 		}
+		root.ctx, root.cancel = context.WithCancel(context.Background())
 		ok := root.Run(tc.desc, tc.f)
 		ctx.release()