internal: testutil: add DevZero helper

This helper acts like /dev/zero (outputs \x00 indefinitely) in an
OS-independent fashion. This ensures we don't need to special-case
around Windows in tests that want to open /dev/zero.

Signed-off-by: Aleksa Sarai <asarai@suse.de>
(cherry picked from commit 2f8d3e1c33)
Signed-off-by: Andrew Hsu <andrewhsu@docker.com>

internal/testutil/helpers.go

@@ -1,6 +1,8 @@
 package testutil
 
 import (
+	"io"
+
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -11,3 +13,15 @@ func ErrorContains(t require.TestingT, err error, expectedError string, msgAndAr
 	require.Error(t, err, msgAndArgs...)
 	assert.Contains(t, err.Error(), expectedError, msgAndArgs...)
 }
+
+// DevZero acts like /dev/zero but in an OS-independent fashion.
+var DevZero io.Reader = devZero{}
+
+type devZero struct{}
+
+func (d devZero) Read(p []byte) (n int, err error) {
+	for i := 0; i < len(p); i++ {
+		p[i] = '\x00'
+	}
+	return len(p), nil
+}