Check in integration test that snapshots are listed before the index

As an exception prune is still allowed to load the index before
snapshots, as it uses exclusive locks. In case of problems with locking
it is also better to load snapshots created after loading the index, as
this will lead to a prune sanity check failure instead of a broken snapshot.

cmd/restic/integration_helpers_test.go

@@ -198,6 +198,9 @@ func withTestEnvironment(t testing.TB) (env *testEnvironment, cleanup func()) {
 		stdout:   os.Stdout,
 		stderr:   os.Stderr,
 		extended: make(options.Options),
+
+		// replace this hook with "nil" if listing a filetype more than once is necessary
+		backendTestHook: func(r restic.Backend) (restic.Backend, error) { return newOrderedListOnceBackend(r), nil },
 	}
 
 	// always overwrite global options