eng: add assertHeap method for memory assertions (#198334)

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

test/unit/electron/renderer.js

@@ -67,6 +67,7 @@ const glob = require('glob');
 const util = require('util');
 const bootstrap = require('../../../src/bootstrap');
 const coverage = require('../coverage');
+const { takeSnapshotAndCountClasses } = require('../analyzeSnapshot');
 
 // Disabled custom inspect. See #38847
 if (util.inspect && util.inspect['defaultOptions']) {
@@ -82,6 +83,7 @@ globalThis._VSCODE_PACKAGE_JSON = (require.__$__nodeRequire ?? require)('../../.
 
 // Test file operations that are common across platforms. Used for test infra, namely snapshot tests
 Object.assign(globalThis, {
+	__analyzeSnapshotInTests: takeSnapshotAndCountClasses,
 	__readFileInTests: path => fs.promises.readFile(path, 'utf-8'),
 	__writeFileInTests: (path, contents) => fs.promises.writeFile(path, contents),
 	__readDirInTests: path => fs.promises.readdir(path),