Specs for where $_ is defined and when it is defined?

Something like:

```ruby
describe "The frame-local variable $_" do
  before :each do
    @name = tmp("frame_local_dollar_under")
    touch(@name) { |f| f.puts "a line" }
  end

  after :each do
    rm_r @name
  end

  it "is set when reading from a file" do
    File.open(@name) do |f|
      $_.should be_nil
      f.gets
      $_.should == "..."
    end
  end
end
```

This is longer but it test exactly the behavior while not interfering with any other and staying consistent. There are likely other behaviors besides setting $_ that will need to be in this describe.