[🚀 Request] Make vscode-extension-tester architecture more flexible

[mlorinc]

Thorough my years working on vscode-extension-tester I have stumbled upon multiple requests about browser and test framework compatibility. Unfortunately, in current state there is very small room for improvements as testing pipeline is fixed and inflexible.

The problem could be possible fixed by following steps below:

1. VSBrowser is mostly OK, just allow users to use browser, so they can test browser VS Code instances + allow them to load custom locators (so they can define own locators for their page objects, e.g. custom WebViews). Create abstract class of Browser, so you can utilise OOP.
2. Create LocatorLoader util, so other people can use custom locators in their tests. I have implemented compatible solution in past.
3. TestRunner needs a lot of work. Pass only Browser instance and test arguments. You can even make abstract class of TestRunner (do not use Mocha in abstract implementation, create specific MochaTestRunner), so it can be used further in ExTester. createShortcut method should not even be there.
4. ExTester should be used as orchestration tools which will be given instances of Browser and TestRunner.

The whole idea is to enable users to build their own pipeline and keep current ExTester/CLI concept. Therefore, users could still use tester as they do now. Users who do not want to use Mocha or native VS Code would be able to create custom runners and reuse page objects and locators. Although, this proposal will require major changes, time investment and modifications to guidelines mentioned above.