go/packages: use t.Context

As it is pointed out in a comment added by CL 204200, let's switch
to t.Context.

Change-Id: I794702e1a7e0755e3fa4a4420f9d0b8919070428
Reviewed-on: https://go-review.googlesource.com/c/tools/+/703076
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Kirill Kolyshkin <kolyshkin@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: kolyshkin

go/packages/packages_test.go

@@ -6,9 +6,7 @@ package packages_test
 
 import (
 	"bytes"
-	"context"
 	"encoding/json"
-	"flag"
 	"fmt"
 	"go/ast"
 	constantpkg "go/constant"
@@ -25,7 +23,6 @@ import (
 	"strings"
 	"testing"
 	"testing/fstest"
-	"time"
 
 	"github.com/google/go-cmp/cmp"
 	"golang.org/x/tools/go/packages"
@@ -36,25 +33,9 @@ import (
 	"golang.org/x/tools/txtar"
 )
 
-// testCtx is canceled when the test binary is about to time out.
-//
-// If https://golang.org/issue/28135 is accepted, uses of this variable in test
-// functions should be replaced by t.Context().
-var testCtx = context.Background()
-
 func TestMain(m *testing.M) {
 	testenv.ExitIfSmallMachine()
 
-	timeoutFlag := flag.Lookup("test.timeout")
-	if timeoutFlag != nil {
-		if d := timeoutFlag.Value.(flag.Getter).Get().(time.Duration); d != 0 {
-			aBitShorter := d * 95 / 100
-			var cancel context.CancelFunc
-			testCtx, cancel = context.WithTimeout(testCtx, aBitShorter)
-			defer cancel()
-		}
-	}
-
 	os.Exit(m.Run())
 }
 
@@ -1924,7 +1905,7 @@ func TestLoadImportsC(t *testing.T) {
 	testenv.NeedsGoPackages(t)
 
 	cfg := &packages.Config{
-		Context: testCtx,
+		Context: t.Context(),
 		Mode:    packages.LoadImports,
 		Tests:   true,
 	}