chore: refactor "exposed" @system_dir instance variable (in helper method)

[pvdb]

It is very unclear why the Rake::TestCase#rake_system_dir helper method:

  def rake_system_dir
    @system_dir = "system"

    FileUtils.mkdir_p @system_dir

    File.write File.join(@system_dir, "sys1.rake"), <<~SYS
      task "sys1" do
        puts "SYS1"
      end
    SYS

    ENV["RAKE_SYSTEM"] = @system_dir
  end

... creates and sets a @system_dir instance variable. 🤔

On inspection, it turns out its only use is the following assertion in one of the test cases:

    assert_equal @system_dir, @app.system_dir

... so this PR removes the ivar and hardcodes the expected "system" value in the assertion:

-    assert_equal @system_dir, @app.system_dir
+    assert_equal "system", @app.system_dir

This may seem a tad controversial, but as it turns out, the sys1.rake Rakefile name and the SYS1 Rakefile contents (also "private" - as it were - to that very same utility method) are already hardcoded in one of the Rake test cases:

$ git grep -i sys1 -- 'test/test*'
test/test_rake_functional.rb:    rake "-g", "sys1"
test/test_rake_functional.rb:    assert_match %r{^SYS1}, @out
test/test_rake_functional.rb:    rake "-g", "sys1", "-T", "extra"
test/test_rake_functional.rb:    rake "sys1", "--trace"
test/test_rake_functional.rb:    assert_match %r{^SYS1}, @out
test/test_rake_functional.rb:    rake "-G", "sys1"
test/test_rake_functional.rb:    rake "--nosearch", "sys1"
test/test_rake_functional.rb:    assert_match %r{^SYS1}, @out
$ _

... so the change in this PR just makes things more consistent with the other test cases and assertions.