How to access a method from app.component from other component?

If you need access to a function from several places, consider putting it in a service as @tibbus mentioned.

utility.service.ts

@Injectable()
export class UtilityService{

    TestingFunction(){}
}

Next, make sure the service is listed in the providers array of your main module:

app.module.ts

// https://angular.io/docs/ts/latest/guide/ngmodule.html#!#ngmodule-properties
@NgModule({ 
  imports:      [ BrowserModule],
  declarations: [ AppComponent, BuyTestComponent ],
  providers:    [ UtilityService ],
  bootstrap:    [ AppComponent ]
})
export class AppModule { }

You can then inject that service in any component that needs access to the function

buy-test.component.ts

@Component(...)
export class BuyTestComponent {

    //inject service into the component
    constructor(private util:UtilityService){}

    TestHere() {
        //access service function
        this.util.TestingFunction();
    }
}