chg: Make set tunnel functions virtual

Author: QEDK

contracts/tunnel/FxBaseChildTunnel.sol

@@ -34,7 +34,7 @@ abstract contract FxBaseChildTunnel is IFxMessageProcessor {
     }
 
     // set fxRootTunnel if not set already
-    function setFxRootTunnel(address _fxRootTunnel) external {
+    function setFxRootTunnel(address _fxRootTunnel) external virtual {
         require(fxRootTunnel == address(0x0), "FxBaseChildTunnel: ROOT_TUNNEL_ALREADY_SET");
         fxRootTunnel = _fxRootTunnel;
     }

contracts/tunnel/FxBaseRootTunnel.sol

@@ -54,7 +54,7 @@ abstract contract FxBaseRootTunnel {
     }
 
     // set fxChildTunnel if not set already
-    function setFxChildTunnel(address _fxChildTunnel) public {
+    function setFxChildTunnel(address _fxChildTunnel) public virtual {
         require(fxChildTunnel == address(0x0), "FxBaseRootTunnel: CHILD_TUNNEL_ALREADY_SET");
         fxChildTunnel = _fxChildTunnel;
     }