This commit fixes the module-level docstrings for the tests and helper
modules in qa. Many of these tests were uncommented previously - this
commit ensures that every test case has at least a minimum level of
commenting.
Adds two RPC tests for the generatetoaddress RPC, one in the wallet, and one when the wallet is disabled.
The wallet RPC Test mines Bitcoin to another node's address and checks that that node has received the Bitcoin.
The RPC test without the wallet mines Bitcoin to an arbitrary address and checks that it works. It then mines to an arbitrary invalid address and checks that that fails.
