mount: enable debug logging for the flaky TestMount test

The test case fails from time to time with an Input/Output error while
trying to access the snapshots directory.

cmd/restic/integration_fuse_test.go

@@ -12,6 +12,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/repository"
 	"github.com/restic/restic/internal/restic"
 	rtest "github.com/restic/restic/internal/test"
@@ -159,6 +160,11 @@ func TestMount(t *testing.T) {
 		t.Skip("Skipping fuse tests")
 	}
 
+	debugEnabled := debug.TestLogToStderr(t)
+	if debugEnabled {
+		defer debug.TestDisableLog(t)
+	}
+
 	env, cleanup := withTestEnvironment(t)
 	// must list snapshots more than once
 	env.gopts.backendTestHook = nil

internal/debug/testing.go

@@ -0,0 +1,23 @@
+package debug
+
+import (
+	"log"
+	"os"
+	"testing"
+)
+
+// TestLogToStderr configures debug to log to stderr if not the debug log is
+// not already configured and returns whether logging was enabled.
+func TestLogToStderr(t testing.TB) bool {
+	if opts.isEnabled {
+		return false
+	}
+	opts.logger = log.New(os.Stderr, "", log.LstdFlags)
+	opts.isEnabled = true
+	return true
+}
+
+func TestDisableLog(t testing.TB) {
+	opts.logger = nil
+	opts.isEnabled = false
+}