Stack_test.rb

RSpec.describe Stack do
  
  before :each do
    @node3 = Node.new(57, nil)
    @node2 = Node.new(21, @node3)
    @node1 = Node.new(34, @node2)
    @stack = Stack.new
  end
  describe "initialize a stack" do
    it "stack.top should eq nil" do
      expect(@stack.top).to eq nil
    end
  end
  
  describe "#push method" do
    it "should set @top to value passed" do
      @stack.push(@node1)
      
      expect(@stack.top).to eq @node1
    end
  end
  describe "#pop method" do
    it "should return last pushed node" do
      @stack.push(@node1)
      
      expect(@stack.pop).to eq @node1
    end
  end
end

nice work. now look up a "before" block in rspec and use a @stack instance variable in that before block to DRY up your tests.