Tests

Depends on #10.

Add tests for:
  * [ ] `Aggregate`
  * [ ] `Append`
  * [ ] `CallbackFilter`
  * [ ] `Counter`
  * [ ] `Demultiplexer`
  * [ ] `Directory`
  * [ ] `Exception`
  * [ ] `FileSystem`
  * [ ] `Filter`
  * [ ] `Glob`
  * [ ] `HasChildren`
  * [ ] `Infinite`
  * [ ] `Iterator`
  * [ ] `IteratorIterator`
  * [ ] `Limit`
  * [ ] `Lookahead`
  * [ ] `Lookbehind`
  * [ ] `Map`
  * [ ] `Mock`
  * [ ] `Multiple`
  * [ ] `NoRewind`
  * [ ] `Outer`
  * [ ] `Recursive/CallbackFilter`
  * [ ] `Recursive/Directory`
  * [ ] `Recursive/Filter`
  * [ ] `Recursive/Iterator`
  * [ ] `Recursive/Lookahead`
  * [ ] `Recursive/Map`
  * [ ] `Recursive/Mock`
  * [ ] `Recursive/Recursive`
  * [ ] `Recursive/RegularExpression`
  * [ ] `Recursive/Tree`
  * [ ] `RegularExpression`
  * [ ] `Repeater`
  * [ ] `Seekable`
  * [ ] `SplFileInfo`