Sample test runner for microsoft/vscode-extension-vscode#157

[octref]

Benefits:

• Test runner no longer coupled with Mocha 4 (options such as useColors are deprecated in Mocha 5)
• Config file such as mocha.opts automatically resolved
• Bugs such as Can't use mocha's dot reporter for extension tests vscode#56211 resolved since test runners should be run in child process. Mocha reporters uses process.stdout which in Extension Host context is not available. Jest / create-react-app uses spawn and AVA need child process as well
• vscode-test doesn't need to depend on mocha. mocha should really be the extension's dev dependency.
• Jest/AVA and other test framework can easily work this way
• A lot more flexibility

I suggest that we standardize the run signature, much as we do for activate of extensions. Its current form has many problems:

function run(testsRoot: string, cb: (error: any, failures: number) => void): void;

• testsRoot is absolute path to the test runner. Not very useful.
• failures has hidden logic for determining success/fail. I suggest plain throw Error for failures.
• 🚨 Unresolved Problem: Previously vscode is available through import in test files. Now since the tests run in another process vscode is not available...Is it possible to support this? If not I'll drop the child_process part, but that takes away all benefits above.

I suggest something like below:

function run(context: vscode.ExtensionTestContext): void;
async function run(context: vscode.ExtensionTestContext): void;

@bpasero What do you think?

[octref]

Can't use vscode here.

[octref]

I can do something like this so everything works: cb83f39 (Following SO answer here: https://stackoverflow.com/a/32945536)

But if the test runner has to contain such workarounds, I would rather encapsulate it in vscode-test and export it, although it adds one more deps and limits the flexibility...

[bpasero]

@octref can I ask why this is a PR for a sample? Is this an example for extensions how to implement a custom test runner?