test(language-service): Move completions test to completions_spec.ts (#33159)

There are many specs in `ts_plugin_spec.ts` that exercise the behavior
of completions. These specs should belong in `completions_spec` instead.

In addition,

1. Tests for `getExternalFiles()` added in `ts_plugin_spec.ts`
2. Fixed bug in MockHost.reset() to remove overriden script names
3. Add test for TS diagnostics when `angularOnly = true` is not set

PR Close #33159

packages/language-service/test/project/app/parsing-cases.ts

@@ -143,10 +143,18 @@ export class ForUsingComponent {
     <div *ngFor="let person of people | async">
       {{person.~{async-person-name}name}}
     </div>
+    <div *ngIf="promisedPerson | async as person">
+      {{person.~{promised-person-name}name}}
+    </div>
   `,
 })
 export class AsyncForUsingComponent {
   people: Promise<Person[]> = Promise.resolve([]);
+  promisedPerson: Promise<Person> = Promise.resolve({
+    name: 'John Doe',
+    age: 42,
+    street: '123 Angular Ln',
+  });
 }
 
 @Component({