Add test cases for backtick fenced code blocks

These test cases are identical to the tilde fenced code blocks. This
also prefixes the original fenced code block test cases with a "Tilde ",
whereas the backtick bordered fenced code block test cases are prefixed,
unsurprisingly, with "Backtick ".
Merge branch 'feature/backtick-bordered-fenced-code-blocks' of git://github.com/ciarand/mdtest
Fixed footnote test case to account for changed output in PHP Markdown Lib.
Additional contrived tests for the fenced block parser.