Revert "testing: add T.Context method"

This reverts commit 26827bc2fe.

Fixes #18199

Change-Id: I42e292cb4e3d740a4fbb5d0380c6ee15ac742092
Reviewed-on: https://go-review.googlesource.com/34141
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/testing/testing_test.go

@@ -5,42 +5,14 @@
 package testing_test
 
 import (
-	"fmt"
 	"os"
-	"runtime"
 	"testing"
-	"time"
 )
 
+// This is exactly what a test would do without a TestMain.
+// It's here only so that there is at least one package in the
+// standard library with a TestMain, so that code is executed.
+
 func TestMain(m *testing.M) {
-	g0 := runtime.NumGoroutine()
-
-	code := m.Run()
-	if code != 0 {
-		os.Exit(code)
-	}
-
-	// Check that there are no goroutines left behind.
-	t0 := time.Now()
-	stacks := make([]byte, 1<<20)
-	for {
-		g1 := runtime.NumGoroutine()
-		if g1 == g0 {
-			return
-		}
-		stacks = stacks[:runtime.Stack(stacks, true)]
-		time.Sleep(50 * time.Millisecond)
-		if time.Since(t0) > 2*time.Second {
-			fmt.Fprintf(os.Stderr, "Unexpected leftover goroutines detected: %v -> %v\n%s\n", g0, g1, stacks)
-			os.Exit(1)
-		}
-	}
-}
-
-func TestContextCancel(t *testing.T) {
-	ctx := t.Context()
-	// Tests we don't leak this goroutine:
-	go func() {
-		<-ctx.Done()
-	}()
+	os.Exit(m.Run())
 }