Stop using math/rand.Read and rand.Seed (deprecated in Go 1.20)

From golangci-lint:

> SA1019: rand.Read has been deprecated since Go 1.20 because it
>shouldn't be used: For almost all use cases, crypto/rand.Read is more
>appropriate. (staticcheck)

> SA1019: rand.Seed has been deprecated since Go 1.20 and an alternative
>has been available since Go 1.0: Programs that call Seed and then expect
>a specific sequence of results from the global random source (using
>functions such as Int) can be broken when a dependency changes how
>much it consumes from the global random source. To avoid such breakages,
>programs that need a specific result sequence should use
>NewRand(NewSource(seed)) to obtain a random generator that other
>packages cannot access. (staticcheck)

See also:

- https://pkg.go.dev/math/rand@go1.20#Read
- https://pkg.go.dev/math/rand@go1.20#Seed

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

snapshots/testsuite/helpers.go

@@ -19,10 +19,10 @@ package testsuite
 import (
 	"context"
 	"fmt"
-	"math/rand"
 	"os"
 
 	"github.com/containerd/containerd/mount"
+	"github.com/containerd/containerd/pkg/randutil"
 	"github.com/containerd/containerd/snapshots"
 	"github.com/containerd/continuity/fs/fstest"
 )
@@ -49,7 +49,7 @@ func applyToMounts(m []mount.Mount, work string, a fstest.Applier) (err error) {
 // createSnapshot creates a new snapshot in the snapshotter
 // given an applier to run on top of the given parent.
 func createSnapshot(ctx context.Context, sn snapshots.Snapshotter, parent, work string, a fstest.Applier) (string, error) {
-	n := fmt.Sprintf("%p-%d", a, rand.Int())
+	n := fmt.Sprintf("%p-%d", a, randutil.Int())
 	prepare := fmt.Sprintf("%s-prepare", n)
 
 	m, err := sn.Prepare(ctx, prepare, parent, opt)