Remove use of es5 multi-line in tests

Some environments don't support multi-lines and it lint errors ([see travis logs](https://travis-ci.org/jashkenas/underscore/builds/34279317)). Its supported by most environments (as far back as IE6 I think) but it can break some parsers and is disallowed in many projects ([Google style guide](http://google-styleguide.googlecode.com/svn/trunk/javascriptguide.xml?showone=Multiline_string_literals#Multiline_string_literals)). I'd say we should just avoid it instead of fixing eslint

/cc @michaelficarra