```ruby
require 'bundler/setup'
require 'fakefs/safe'
require 'maxitest/autorun'

module Minitest
  class Test
    # Minitest::Test#diff needs to write to the filesystem in order to produce
    # the nice diffs we see when a test fails. For this to work it needs to
    # access the real filesystem.
    def diff(expected, actual)
      act_on_real_fs { super(expected, actual) }
    end

    def act_on_real_fs(&block)
      FakeFS.without(&block)
    end

    def capture_stderr
      real_stderr, $stderr = $stderr, StringIO.new

      # force FileUtils to use our stderr
      RealFileUtils.instance_variable_set('@fileutils_output', $stderr)

      yield

      $stderr.string
    ensure
      $stderr = real_stderr

      # restore FileUtils stderr
      RealFileUtils.instance_variable_set('@fileutils_output', $stderr)
    end

    def real_file_sandbox(path = nil)
      base_path = real_file_sandbox_path
      path ? File.join(base_path, path) : base_path
    end

    def real_file_sandbox_path
      File.expand_path(File.join(File.dirname(__FILE__), '..', 'test_sandbox'))
    end

    def teardown
      return if !FakeFS.activated?
      FakeFS.deactivate!
      flunk "always deactivate FakeFs after test run"
    end
  end
end
```

Fix tiny rubocop violation in test using unless for negative condition
user
```ruby
require 'bundler/setup'
require 'fakefs/safe'
require 'maxitest/autorun'

module Minitest
  class Test
    # Minitest::Test#diff needs to write to the filesystem in order to produce
    # the nice diffs we see when a test fails. For this to work it needs to
    # access the real filesystem.
    def diff(expected, actual)
      act_on_real_fs { super(expected, actual) }
    end

    def act_on_real_fs(&block)
      FakeFS.without(&block)
    end

    def capture_stderr
      real_stderr, $stderr = $stderr, StringIO.new

      # force FileUtils to use our stderr
      RealFileUtils.instance_variable_set('@fileutils_output', $stderr)

      yield

      $stderr.string
    ensure
      $stderr = real_stderr

      # restore FileUtils stderr
      RealFileUtils.instance_variable_set('@fileutils_output', $stderr)
    end

    def real_file_sandbox(path = nil)
      base_path = real_file_sandbox_path
      path ? File.join(base_path, path) : base_path
    end

    def real_file_sandbox_path
      File.expand_path(File.join(File.dirname(__FILE__), '..', 'test_sandbox'))
    end

    def teardown
      return unless FakeFS.activated?
      FakeFS.deactivate!
      flunk "always deactivate FakeFs after test run"
    end
  end
end
```
assistant