test(language-service): Remove all markers from test project (#37475)

This commit removes all markers from the inline template in
`AppComponent` and external template in `TemplateReference`.

Test scenarios should be colocated with the test cases themselves.
Besides, many existing cases are invalid. For example, if we want to
test autocomplete for HTML element, the existing test case is like:
```
<~{cursor} h1>
```
This doesn't make much sense, becasue the language service already sees
the `h1` tag in the template. The correct test case should be:
```
<~{cursor
```
IMO, this reflects the real-world use case better.

This commit also uncovers a bug in the way HTML entities autocompletion
is done. There's an off-by-one error in which a cursor that immediately
trails the ampersand character fails to trigger HTML entities
autocompletion.

PR Close #37475

packages/language-service/test/project/app/app.component.ts

@@ -15,16 +15,9 @@ export interface Hero {
 
 @Component({
   selector: 'my-app',
-  template: `~{empty}
-    <~{start-tag}h~{start-tag-after-h}1~{start-tag-h1} ~{h1-after-space}>
-      ~{h1-content} {{~{sub-start}title~{sub-end}}}
-    </h1>
-    ~{after-h1}<h2>{{~{h2-hero}hero.~{h2-name}name}} details!</h2>
-    <div><label>id: </label>{{~{label-hero}hero.~{label-id}id}}</div>
-    <div ~{div-attributes}>
-      <label>name: </label>
-    </div>
-    &~{entity-amp}amp;
+  template: `
+    <h1>{{title}}</h1>
+    <h2>{{hero.name}} details!</h2>
   `
 })
 export class AppComponent {

packages/language-service/test/project/app/test.ng

@@ -1,7 +1,2 @@
-~{empty}
-<~{start-tag}h~{start-tag-after-h}1~{start-tag-h1} ~{h1-after-space}>
-  ~{h1-content} {{~{sub-start}title~{sub-end}}}
-</h1>
-~{after-h1}<h2>{{~{h2-hero}hero.~{h2-name}name}} details!</h2>
-<div><label>id: </label>{{~{label-hero}hero.~{label-id}id}}</div>
-&~{entity-amp}amp;
+<h1>{{title}}</h1>
+<h2>{{hero.name}} details!</h2>