steps_test.rb

gistfile1.rb

class StepTest < ActiveSupport::TestCase

  test "create step with unexistant office" do
    step = Step.create name: 'Computo'
    assert step.office.name == 'Computo'
  end
  
  test "create step with existant office" do
    office = offices :computo
    step = Step.create office: office
    assert step.office.name == 'Computo'
  end
  
end

no sería asi el test del controller?

require 'test_helper'

class StepsControllerTest < ActionController::TestCase

  test "move file record through offices" do
    file_record = file_records(:legal)
    office = offices :computo
    step = {office_id: office.id}
    post :create, step: step, file_record_id: file_record.id
    assert file_record.steps.last.office.name == 'Computo'
  end
  
end

si no te anda, probá con esta modificación

assert file_record.steps(true).last.office.name == 'Computos'

cuando a la relación le mandás true, vuelve a hacer la consulta. o sea, la actualiza.