Fix changing notebook Python intepreter also changing console

[seeM]

Addresses #5305. I also added unit tests for the Python runtime session class. They're not necessary for this specific change, but the setup will be useful moving forward.

[nstrayer]

Tested locally and works as advertised. I couldn't actually figure out how to run the unit test.

[nstrayer]

Oh I think we need to add

        // ...
	{
		label: 'positron-python',
		workspaceFolder: path.join(os.tmpdir(), `positron-python-${Math.floor(Math.random() * 100000)}`),
		mocha: { timeout: 60_000 }
	}
	// ...

to .vscode-test.js to run the tests with yarn test-extension -l positron-python

[seeM]

Ah, the Python extension's tests are a bit complex and run differently to the others. You can run the unit tests with yarn test:unittests from the extension folder.