Support for alternate JupyterLab distributions

[jtpio]

Problem

It would be great if galata could also be used to test alternative JupyterLab distributions such as RetroLab.

Proposed Solution

At the moment it looks like the default page hits the /lab endpoint:

galata/packages/galata/jest-env.js

Line 179 in ba2713f

let jlabUrl = path.join(baseUrl, 'lab');

There is also this setup logic using /lab by default:

galata/packages/galata/jest-env.js

Lines 143 to 149 in ba2713f

	let resourcePath = '/lab';
	if (context.jlabWorkspace) {
	resourcePath = `${resourcePath}/workspaces/${context.jlabWorkspace}`;
	}
	await context.page.evaluate(async resourcePath => {
	await window.galataip.waitForLaunch(resourcePath);
	}, resourcePath);

Additional context

A potential candidate for this would be RetroLab: jupyterlab/retrolab#135