test(material/legacy-chips): fix selectable chips tests (#25790)

Fixes that we had some tests where we were testing user selection of a chip, but the chip wasn't set as selectable.

Author: crisbeto

src/material/legacy-chips/chip-list.spec.ts

@@ -792,6 +792,7 @@ describe('MatChipList', () => {
       });
 
       it('should take an initial view value with reactive forms', () => {
+        fixture.componentInstance.selectable = true;
         fixture.componentInstance.control = new FormControl('pizza-1');
         fixture.detectChanges();
 
@@ -995,6 +996,7 @@ describe('MatChipList', () => {
       });
 
       it('should take an initial view value with reactive forms', () => {
+        fixture.componentInstance.selectable = true;
         fixture.componentInstance.control = new FormControl(['pizza-1']);
         fixture.detectChanges();