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

aio/content/guide/testing.md

@@ -2367,9 +2367,9 @@ The [override metadata object](#metadata-override-object) is a generic defined a
 
 <code-example format="." language="javascript">
   type MetadataOverride<T> = {
-    add?: T;
-    remove?: T;
-    set?: T;
+    add?: Partial<T>;
+    remove?: Partial<T>;
+    set?: Partial<T>;
   };
 </code-example>
 
@@ -2725,9 +2725,9 @@ appropriate to the method, that is, the parameter of an `@NgModule`,
 
 <code-example format="." language="javascript">
   type MetadataOverride<T> = {
-    add?: T;
-    remove?: T;
-    set?: T;
+    add?: Partial<T>;
+    remove?: Partial<T>;
+    set?: Partial<T>;
   };
 </code-example>
 

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);

tools/public_api_guard/core/testing.d.ts

@@ -55,9 +55,9 @@ export declare class InjectSetupWrapper {
 
 /** @experimental */
 export declare type MetadataOverride<T> = {
-    add?: T;
-    remove?: T;
-    set?: T;
+    add?: Partial<T>;
+    remove?: Partial<T>;
+    set?: Partial<T>;
 };
 
 /** @experimental */