fix(@angular-devkit/schematics): ensure collections can be resolved via test runner in pnpm workspaces

Currently when operating within a pnpm workspace and leveraging the
schematic test runner, there are situations where e.g.
`@schematics/angular` cannot be resolved. Consider this pnpm node
modules structure:

```
packages/
  pwa/
    node_modules/@schematics/angular --> .pnpm-store/@schematics/angular/...
    node_modules/@angular-devkit/schematics --> .pnpm-store/@angular-devkit/schematics/...
    index_spec.js // trying to call external schematic `@schematics/angular`
```

This above setup will fail because `@schematics/angular` is attempted to
be resolved from within the devkit schematics code, which doesn't have
access, or a dependency on `@schematics/angular`. We can use the
specified collection of the test runner to determine a good "resolution
lookup site", similiar to how it happens with the real `ng update`
command.

Author: devversion

packages/angular_devkit/schematics/testing/schematic-test-runner.ts

@@ -43,15 +43,23 @@ export class UnitTestTree extends DelegateTree {
 }
 
 export class SchematicTestRunner {
-  private _engineHost = new NodeModulesTestEngineHost();
-  private _engine: SchematicEngine<{}, {}> = new SchematicEngine(this._engineHost);
+  private _engineHost: NodeModulesTestEngineHost;
+  private _engine: SchematicEngine<{}, {}>;
   private _collection: Collection<{}, {}>;
   private _logger: logging.Logger;
 
   constructor(
     private _collectionName: string,
     collectionPath: string,
   ) {
+    this._engineHost = new NodeModulesTestEngineHost([
+      // Leverage the specified collection path as an additional base for resolving other
+      // collections by name. This is useful in e.g. pnpm workspaces where `@angular-devkit/schematics`
+      // doesn't necessarily have access to e.g. `@schematics/angular`.
+      collectionPath,
+    ]);
+    this._engine = new SchematicEngine(this._engineHost);
+
     this._engineHost.registerCollection(_collectionName, collectionPath);
     this._logger = new logging.Logger('test');