Add functional tests for template rendering
user
```ruby
require 'spec_helper'

module Omnibus
  class RandomClass
    include Templating
  end

  describe Templating do
    subject { RandomClass.new }

    describe '#render_template' do
      let(:source)      { "#{tmp_path}/source.erb" }
      let(:destination) { "#{tmp_path}/final" }
      let(:mode)        { 0644 }
      let(:variables)   { { name: 'Name' } }
      let(:contents) do
        <<-EOH.gsub(/^ {10}/, '')
          <%= name %>

          <% if false -%>
            This is magic!
          <% end -%>
        EOH
      end

      let(:options) do
        {
          destination: destination,
          variables:   variables,
          mode:        mode,
        }
      end

      before do
        File.open(source, 'w') { |f| f.write(contents) }
      end

      context 'when no destination is given' do
        let(:destination) { nil }

        it 'renders adjacent, without the erb extension' do
          subject.render_template(source, options)
          expect("#{tmp_path}/source").to be_a_file
        end
      end

      context 'when a destination is given' do

        it 'renders at the destination' do
          subject.render_template(source, options)
          expect(destination).to be_a_file
        end
      end

      context 'when a mode is given' do
        let(:mode) { 0755 }

        it 'renders the object with the mode' do
          subject.render_template(source, options)
          expect(destination).to be_an_executable
        end
      end

      context 'when an undefined variable is used' do
        let(:contents) { "<%= not_a_real_variable %>" }

        it 'raise an exception' do
          expect { subject.render_template(source, options) }.to raise_error
        end
      end
    end
  end
end
```
assistant