encoding/json, internal/testenv: use Fatalf

Change-Id: I64dd09e76d811000a914776fdad47808e3895690
Reviewed-on: https://go-review.googlesource.com/20989Reviewed-by: Dave Cheney <dave@cheney.net>

src/encoding/json/decode_test.go

@@ -1270,7 +1270,7 @@ func TestSliceOfCustomByte(t *testing.T) {
 		t.Fatal(err)
 	}
 	if !reflect.DeepEqual(a, b) {
-		t.Fatal("expected %v == %v", a, b)
+		t.Fatalf("expected %v == %v", a, b)
 	}
 }
 

src/internal/testenv/testenv.go

@@ -74,7 +74,7 @@ func GoToolPath(t *testing.T) string {
 	}
 	goBin, err := exec.LookPath("go" + exeSuffix)
 	if err != nil {
-		t.Fatal("cannot find go tool: %v", err)
+		t.Fatalf("cannot find go tool: %v", err)
 	}
 	return goBin
 }