[#326] pool: Add test for concurrent client deletion

Signed-off-by: Alex Vanin <a.vanin@yadro.com>

pool/tree/pool_test.go

@@ -421,14 +421,15 @@ func TestDeleteClientTwice(t *testing.T) {
 	p := Pool{
 		clientMap: makeClientMap([]netmap.NodeInfo{netmaptest.NodeInfo()}),
 	}
-	// emulate concurrent request as consecutive requests
-	// to delete the same client id from the map twice
+	// emulate concurrent requests as consecutive requests
+	// to delete the same client from the map twice
 	for idToDelete := range p.clientMap {
 		p.deleteClientFromMap(idToDelete)
 		require.NotPanics(t, func() {
 			p.deleteClientFromMap(idToDelete)
 		})
 	}
+	require.Empty(t, p.clientMap)
 }
 
 func makeInnerPool(nodes [][]string) []*innerPool {