Add more test coverage

[twmarshall]

We've covered most of the basic testing in the initial patch, but there are a few things we haven't gotten to and in the interest of getting the initial patch finalized we're going to postpone those things and write a follow up patch with them later.

Some known things that need testing:

• thrift timeouts
• conf_overlay parameter to HS2Session::ExecuteStatement
• GetFunctions operation
• different data types (currently we only fetch strings and ints)
• various error conditions, such as trying to use invalid session and operation handles