Change correctness to use pytests

[apkar]

Correctness implements unit tests with some custom framework. There would be a lot of benefits if we use a standard testing framework

• Easier for new developers with a standard framework
• Framework like PyTest can also manage dependencies like fdbserver and fdbdoc processes
• We can use asserts instead of passing around the error code. Tests would be more readable.

Subtasks

• [x] Simple PyTest fixture for collection management - #83
• [x] Move unit tests onto PyTest - #83
• [x] CI should run PyTest based tests - #83
• [ ] Add a fixture to start DocLayer instance
• [ ] Add a fixture to download and start FoundationDB instance
• [ ] Move randomized correctness onto PyTest