This adds an `assertHeap` function that can be used in tests. It
takes a heap snapshot, and asserts the state of classes in memory. This
works in Node and the Electron sandbox, but is a no-op in the browser.
Snapshots are process asynchronously and will report failures at the end
of the suite.
This method should be used sparingly (e.g. once at the end of a suite to
ensure nothing leaked before), as gathering a heap snapshot is fairly
slow, at least until V8 11.5.130 (https://v8.dev/blog/speeding-up-v8-heap-snapshots).
When used, the function will ensure the test has a minimum timeout
duration of 20s to avoid immediate failures.
It takes options containing a mapping of class names, and assertion functions
to run on the number of retained instances of that class. For example:
```ts
assertSnapshot({
classes: {
ShouldNeverLeak: count => assert.strictEqual(count, 0),
SomeSingleton: count => assert(count <= 1),
}
});
```
Closes https://github.com/microsoft/vscode/issues/191920
