docs: testing guide for CLI (#20697)

- updates tests
- heavy prose revisions
- uses HttpClient (with angular-in-memory-web-api)
- test HeroService using `HttpClientTestingModule`
- scrub away most By.CSS
- fake async observable with `asyncData()`
- extensive Twain work
- different take on retryWhen
- remove app barrels (& systemjs.extras) which troubled plunker/systemjs
- add dummy export const to hero.ts (plunkr/systemjs fails w/o it)
- shrink and re-organize TOC
- add marble testing package and tests
- demonstrate the "no beforeEach()" test coding style
- add section on Http service testing
- prepare for stackblitz
- confirm works in plunker except excluded marble test
- add tests for avoidFile class feature of CodeExampleComponent

PR Close #20697

aio/tools/stackblitz-builder/builder.js

@@ -19,9 +19,10 @@ class StackblitzBuilder {
     var packageJson = require(path.join(__dirname, '../examples/shared/boilerplate/cli/package.json'));
     this.examplePackageDependencies = packageJson.dependencies;
 
-    // Add jasmine-core (which is a devDependency) for unit test examples.
+    // Add unit test packages from devDependency for unit test examples
     var devDependencies = packageJson.devDependencies;
     this.examplePackageDependencies['jasmine-core'] = devDependencies['jasmine-core'];
+    this.examplePackageDependencies['jasmine-marbles'] = devDependencies['jasmine-marbles'];
 
     this.copyrights = {};