Update README.md (#1240)

add vlan package for the command line vconfig

Author: qiluo-msft

tests/README.md

@@ -13,7 +13,7 @@ SWSS, Redis, and all the other required components run inside a virtual switch D
 
     ```
     sudo modprobe team
-    sudo apt install net-tools ethtool
+    sudo apt install net-tools ethtool vlan
     sudo pip install docker zipp==2.2.1 pytest==4.6.9 flaky redis
     ```
 3. Install `python-swsscommon_1.0.0_amd64.deb`. You will need to install all the dependencies as well in the following order: