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>
 };