feat(@angular-devkit/schematics): Add registerCollection to SchematicsTestRunner

Fixes #14833

Author: ukrukarg

etc/api/angular_devkit/schematics/testing/index.d.ts

@@ -4,6 +4,7 @@ export declare class SchematicTestRunner {
     readonly tasks: TaskConfiguration[];
     constructor(_collectionName: string, collectionPath: string);
     callRule(rule: Rule, tree: Tree, parentContext?: Partial<SchematicContext>): Observable<Tree>;
+    registerCollection(collectionName: string, collectionPath: string): void;
     runExternalSchematic<SchematicSchemaT>(collectionName: string, schematicName: string, opts?: SchematicSchemaT, tree?: Tree): UnitTestTree;
     runExternalSchematicAsync<SchematicSchemaT>(collectionName: string, schematicName: string, opts?: SchematicSchemaT, tree?: Tree): Observable<UnitTestTree>;
     runSchematic<SchematicSchemaT>(schematicName: string, opts?: SchematicSchemaT, tree?: Tree): UnitTestTree;