Addition & modification of instructions to run tests.

[Zshan0]

The PR updates the current instructions to run revaultd tests in the following way:

• bitcoind requirement is implicit and is not mentioned in the README, the requirement is made explicit
• The source code already contains a script for compiling the test servers, mentioning it would make it easier for contributors & users.
• The docker requirement is mentioned in coordinatord which also can be added here to avoid confusion.
• There is a chance that older or less powerful systems will encounter timeout errors, stating a simple way to avoid them will make debugging easier