Tests

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