Added util.py, includes source directory import and TestWrapper class.

[jachiang]

This PR moves the TestWrapper class from the bitcoin core source directory to the taproot-workhop repository. The bitcoin source directory path import has also been consolidated here.

Users now can do this:

• Edit source directory in util.py
• Run import util
• Run test = util.TestWrapper()

This PR depends on:
The refactor of main in BitcoinTestFramework, which is merged into the bitcoin/optech-taproot branch. The refactor spilts out setup and shutdown from main so these methods are available to child classes, such as TestWrapper. This has been merged in optech-taproot-develop and tagged v0.1.

TODO's:

• Implement singleton pattern during startup to ensure not other bitcoind processes are running, with a cross-platform compatible-way of acquiring PIDs.

[bitschmidty]

Im not strong on best practices for keeping user config values separate from other utility functions. Could keep as setup.py and somehow reference path from util.py, but could be awkward.

[jachiang]

Yeah, the only reason source dir path import and TestWrapper are consolidated in util.py is because I wanted to avoid to reference path from util.py, after setup.py sets source directory.

[bitschmidty]

Might note here that the value of this class instead of just using BitcoinTestFramework is that weve provided a bunch of sane default settings as well as defaulting to setup 3 nodes. (at least that is my understanding of the raison d'etre for this class)

[jachiang]

Yes, BitcoinTestFramework sets defaults when reading command line arguments with argparse. Since we don't do that, we have to set the defaults in TestWrapper. Only when these options are set can we run the setup() and shutdown().

[jachiang]

@jnewbery I am unsure of how best to implement the singleton pattern and detect other bitcoind instances. pidof isn't necessarily installed on OSX. Windows requires a different utility. psutil is cross-platform but requires a package install. If no obvious solutions, I suggest to hold off on this for now, since startup of a TestWrapper already fails if another instance already running.

I would like to clarify how to move forward on this TestWrapper, since all other notebook pages depend on this PR.

[jnewbery]

I'm going to merge this now. We can do a few tidy-ups to this later, but it's important we get this in so that the individual notebooks can be built on top of it.