a.rb

require 'tempfile'

# File.openを使う処理
def write_hello(path)
  File.open(path, "a") do |file|
    file.write "hello"
  end
end

# テスト
def test_write_msg
  tempfile = Tempfile.new
  tempfile.close

  write_hello(tempfile.path)

  raise "テスト失敗" if File.read(tempfile.path) != "hello"
end

test_write_msg

b.rb

require 'tempfile'

class A
  attr_accessor :file_path

  # File.openを使う処理
  def write_hello
    File.open(@file_path, "a") do |file|
      file.write "hello"
    end
  end
end

# テスト
def test_write_msg
  a = A.new
  tempfile = Tempfile.new
  tempfile.close

  a.file_path = tempfile.path
  a.write_hello

  raise "テスト失敗" if File.read(tempfile.path) != "hello"
end

test_write_msg