refactor(core): use Partial<T> for MetadataOverride (#24103)

Allows to write:

const fixture = TestBed
      .overridePipe(DisplayNamePipe, { set: { pure: false } })
      .createComponent(MenuComponent);

when you only want to set the `pure` metadata,
instead of currently:

const fixture = TestBed
      .overridePipe(DisplayNamePipe, { set: { name: 'displayName', pure: false } })
      .createComponent(MenuComponent);

which forces you to redefine the name of the pipe even if it is useless.

Fixes #24102

PR Close #24103

packages/core/testing/src/metadata_override.ts

@@ -12,7 +12,7 @@
  * @experimental
  */
 export type MetadataOverride<T> = {
-  add?: T,
-  remove?: T,
-  set?: T
+  add?: Partial<T>,
+  remove?: Partial<T>,
+  set?: Partial<T>
 };

packages/platform-browser/test/testing_public_spec.ts

@@ -376,7 +376,8 @@ class CompWithUrlTemplate {
             TestBed
                 .overrideComponent(
                     SomeComponent, {set: {selector: 'comp', template: `{{'hello' | somePipe}}`}})
-                .overridePipe(SomePipe, {set: {name: 'somePipe'}});
+                .overridePipe(SomePipe, {set: {name: 'somePipe'}})
+                .overridePipe(SomePipe, {add: {pure: false}});
           });
           it('should work', () => {
             const compFixture = TestBed.createComponent(SomeComponent);