Strings: added tests for more UTF-8 edge cases

I looked into some UTF-8 / Unicode edge cases. 

* All UTF-8 related functions operate to code point level. I mentioned it explicitly in `Strings::length` phpDoc. Should we add similar note to all methods or maybe to just to class phpDoc?
* `iconv_substr` handles negative length + negative offset [differently](http://3v4l.org/JvOlj) than `mb_substr`, therefore the new `substring` test case will fail when `mb_substr` is not available. Suggestions?
* Results of "3.2 Lonely start characters" is inconsistent and a bit strange. Maybe we should just check that the output consists only of spaces?

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Based on Travis results is `Strings::chr` and `Strings::toAscii` behavior also inconsistent.