snapshots: fix flaky tests (#8475)

The use testing.T.TempDir() seems to cause test failures
in CI environvements in those cases where temporary
directories' subdirs are created with permissions that
are different from the defaults used by testing.T.TempDir().
The snapshots package's tests seem to be heavily affected,
thus this replaces testing.T.TempDir() occurrences with
ioutil.TempDir() calls.

Related upstream issue:
- golang/go#40853

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

snapshots/helpers_test.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"io"
 	"io/ioutil"
+	"os"
 	"testing"
 	"time"
 
@@ -100,7 +101,10 @@ func (m *mockSnapshotter) Snapshot(height uint64, format uint32) (<-chan io.Read
 // setupBusyManager creates a manager with an empty store that is busy creating a snapshot at height 1.
 // The snapshot will complete when the returned closer is called.
 func setupBusyManager(t *testing.T) *snapshots.Manager {
-	tempdir := t.TempDir()
+	tempdir, err := ioutil.TempDir("", "")
+	require.NoError(t, err)
+	t.Cleanup(func() { _ = os.RemoveAll(tempdir) })
+
 	store, err := snapshots.NewStore(db.NewMemDB(), tempdir)
 	require.NoError(t, err)
 	hung := newHungSnapshotter()

snapshots/store_test.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"io"
 	"io/ioutil"
+	"os"
 	"path/filepath"
 	"testing"
 	"time"
@@ -19,7 +20,10 @@ import (
 )
 
 func setupStore(t *testing.T) *snapshots.Store {
-	tempdir := t.TempDir()
+	tempdir, err := ioutil.TempDir("", "")
+	require.NoError(t, err)
+	t.Cleanup(func() { _ = os.RemoveAll(tempdir) })
+
 	store, err := snapshots.NewStore(db.NewMemDB(), tempdir)
 	require.NoError(t, err)