bytes: skip TestNewBufferShallow if optimization is disabled

TestNewBufferShallow should be skipped if optimization is disabled.

It is currently failing on no-opt builders.

Change-Id: Ib5e62022a56a4e5f158f247d69a6229d2cb4d99e
Reviewed-on: https://go-review.googlesource.com/c/go/+/581915
Auto-Submit: Than McIntosh <thanm@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Quim Muntal <quimmuntal@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Joedian Reid <joedian@google.com>
diff --git a/src/bytes/buffer_test.go b/src/bytes/buffer_test.go
index 322e736..3c964fc 100644
--- a/src/bytes/buffer_test.go
+++ b/src/bytes/buffer_test.go
@@ -7,6 +7,7 @@
 import (
 	. "bytes"
 	"fmt"
+	"internal/testenv"
 	"io"
 	"math/rand"
 	"strconv"
@@ -100,6 +101,7 @@
 // should not result in any allocations.
 // This can be used to reset the underlying []byte of an existing Buffer.
 func TestNewBufferShallow(t *testing.T) {
+	testenv.SkipIfOptimizationOff(t)
 	n := testing.AllocsPerRun(1000, func() {
 		buf = *NewBuffer(testBytes)
 	})