Add unit tests for MigrationDumper.
user
```ruby
require 'stringio'
require 'helper'

class MigrationDumperTest < TestCase
  def test_without_calls
    create_dumper

    assert_change <<-STR
    STR
  end

  def test_create_table_call
    create_dumper \
      create_call(:create_table, "table", :force => true) { |t|
        t.string  "uid", :limit => 32, :unique => true
        t.integer "amount"
      }

    assert_change <<-STR
      create_table "table", :force => true do |t|
        t.string "uid", :limit => 32, :unique => true
        t.integer "amount"
      end
    STR
  end

  def test_index_call
    create_dumper \
      create_call(:add_index, "table", "uid", :unique => true),
      create_call(:add_index, "table", %w[composite index])

    assert_change <<-STR
      add_index "table", "uid", :unique => true
      add_index "table", ["composite", "index"]
    STR
  end

  private

  def create_dumper(*calls)
    @dumper = Unschema::MigrationDumper.new("foo_bar", calls)
  end

  def create_call(name, *args, &block)
    Unschema::SchemaIntermediator::Call.new(name, *args, &block)
  end

  def assert_change(inner)
    expect = <<-STR
      class CreateFooBar < ActiveRecord::Migration
        def change
    STR

    unless inner.empty?
      expect << inner.unindent.indent(expect.indentation + 4)
    end

    expect << <<-STR
        end
      end
    STR

    assert_dump expect
  end

  def assert_dump(expect)
    io = StringIO.new
    @dumper.dump_to(io)
    assert_string io.string, expect
  end
end
```
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