Fix test suite compatibility with Spring 2.1.0+.
    
    Spring 2.1.0+ moved the spring/test to be part of the test suite [1].
    Use the necessary bits as long as spring-watcher-listen is not fixed.
    
    Please note that the test suite archive is reused from the
    rubygem-spring.
    
    I have not bothered to report this issue becasue:
    
    * The upstream appers to be rather dead, because there are already other
      pending requestes such as [2, 3].
    * spring-watcher-listen dependency was dropped from Rails [4] due to
      some issues, so it will be probably better to drop this package in the
      near future.
    
    [1] https://github.com/rails/spring/commit/cfc0c5bce2a1f7da4bfc03d3b0146ab797a9798a
    [2] https://github.com/jonleighton/spring-watcher-listen/pull/22
    [3] https://github.com/jonleighton/spring-watcher-listen/issues/17
    [4] https://github.com/rails/rails/pull/36377