DP: TN: Update Testnet info page

[relativityboy]

As a developer I want current information about setting up and connecting to the testnet. Update the test information to be current with HF20 so that developers can work with current testnets.

AC

• [x] testnet information is updated so devs can build testnets and understand how our current testnet setup works.
• [x] describe how Steemit's main testnet is different from Mainnet in base behaviors, and how to determine some of those differences (aka how to examine a Steem testnet for various attributes). Include gatling and bootstrap (initial state)

[inertia186]

In the standup, we opted for minor updates only.