こんなテストが通るような実装を考えている

context_block.rb

require 'rspec'

describe do
  it do
    ary = [:one, :two]
    context(ary) do
      send(:[],0)
    end.should == :one
  end
  it do
    ary = [:one, :two]
    context(ary) do
      send(:[],1)
    end.should == :two
  end
end