Unit tests for several SmartList slice bugs

[nyurik]

Update smart list unit tests to highlight several iteration and slicing related bugs. Many of the tests are ran twice - once comparing a list, and once iterating over the items - for some reason behavior is not the same.

Note that some of the unit test also runs against the built-in list object to make sure behavior is consistent.

See https://github.com/earwig/mwparserfromhell/issues/226