[tests] Add browser test for real-time updates

[nemesifier]

While manually testing #249, I realized that having browser tests for the following actions would help ease maintenance:

• Changing the link status and verifying that the link changes color
• Adding/removing a node and verifying that it is added/removed from the graph
• Adding/removing a link and verifying that it is added/removed from the graph

This is somewhat related to #250.