storage: retry failed liveness heartbeats in tests

Teach multiTestContext.heartbeatLiveness to retry heartbeats that fail
due to ErrEpochIncremented. This error is expected when the heartbeat
races with an epoch increment from another store.

Fix cockroachdb#27835.

Release note: None

pkg/storage/client_test.go

@@ -1207,7 +1207,13 @@ func (m *multiTestContext) heartbeatLiveness(ctx context.Context, store int) err
 	if err != nil {
 		return err
 	}
-	return nl.Heartbeat(ctx, l)
+
+	for r := retry.StartWithCtx(ctx, retry.Options{MaxRetries: 5}); r.Next(); {
+		if err = nl.Heartbeat(ctx, l); err != storage.ErrEpochIncremented {
+			break
+		}
+	}
+	return err
 }
 
 // advanceClock advances the mtc's manual clock such that all