Add Honeybadger.synchronize to synchronize with the agent.

This allows tests to make assertions on the test backend without
worrying about race conditions. Example:

  it "sends notifications to Honeybadger" do
    expect {
      Honeybadger.synchronize do
        50.times do
          Honeybadger.notify(StandardError.new('test backend'))
        end
      end
    }.to change(Honeybadger::Backend::Test.notifications[:notices], :size).by(50)
  end