Why rails minitest testcases are failing

My posts_controller_Test.rb file:

# test/controllers/posts_controller_test.rb
require 'test_helper'

class PostControllerTest < ActionController::TestCase
    def setup
       @post = Post.new
    end

    test 'should get index' do
        get :index
        assert_response :success
        assert_not_nil assigns(:posts)
    end
end

My post_controller File

class PostsController < ActionController::Base
    before_action :authenticate_user!

    def index
        @post = current_user.posts.paginate(page: params[:page], per_page: 5)
        respond_to do |format|
            format.html
            format.json { render json: @post }
        end
    end

    def new
        @post = Post.new
    end

    def create
        @post = current_user.posts.build(post_param)
        if @post.save
            redirect_to action: 'index'
        else
            render 'new'
    end
end

Why my test cases are failing? Is it because I have authenticate_user! condition? I also have .yml file and tried to test with it but after initializing with .yml data I am getting RuntimeError: @controller is nil: make sure you set it in your test's setup method.

yml file

one:
 data 2
 value: 3
 user_id: 1
 name: 'test'

.yml has everything what i have required in

params.require(:post).permit(:data, :value, :name) and obvious `user` for foreign key reference

Edit -1

After the suggestion of inheriting with ApplicationController

Got this new error:

 NameError: uninitialized constant ApplicationController::Base