Fix TestRenderingFootnotes for HTML5 validation

[anthonyfok]

Hello,

Blackfriday v1.5.1 (and v2 HEAD) removed rel="footnote" to fix HTML5 validation on validator.w3.org.

See russross/blackfriday#433 for more information.

This was discovered in the process of fixing gohugoio/hugo#4350 when blackfriday v1.5.1, which hugo v0.37 requires, failed to pass the Ubuntu Autopkgtest with golang-github-chaseadamsio-goorgeous 2.0.0+git20171126.dcf1ef8-1 at http://autopkgtest.ubuntu.com/packages/g/golang-github-chaseadamsio-goorgeous/bionic/amd64:

=== RUN   TestRenderingFootnotes
--- FAIL: TestRenderingFootnotes (0.00s)
    goorgeous_test.go:431: simple: Footnote for Org() from: 
         Test 1[fn:1] and Test 2[fn: 2] and Test 3[fn:3] also test lettres[fn:let] then final test[fn:4]

        [fn:let] what?

        [fn:6] And test it[fn:7].

        * Footnotes

        [fn:1] Test 1

        [fn:3] Test 3

        [fn:2] Test2

        [fn:5] missing?

        [fn:6] Six?

        [fn:7] Seven?? 
         result: 
          <p>Test 1<sup class="footnote-ref" id="fnref:1"><a href="#fn:1">1</a></sup> and Test 2[fn: 2] and Test 3<sup class="footnote-ref" id="fnref:3"><a href="#fn:3">2</a></sup> also test lettres<sup class="footnote-ref" id="fnref:let"><a href="#fn:let">3</a></sup> then final test<sup class="footnote-ref" id="fnref:4"><a href="#fn:4">4</a></sup></p>

        <h1 id="footnotes">Footnotes</h1>
        <div class="footnotes">

        <hr />

        <ol>
        <li id="fn:1">Test 1 <a class="footnote-return" href="#fnref:1"><sup>↩</sup></a></li>

        <li id="fn:3">Test 3 <a class="footnote-return" href="#fnref:3"><sup>↩</sup></a></li>

        <li id="fn:let">what? <a class="footnote-return" href="#fnref:let"><sup>↩</sup></a></li>

        <li id="fn:4">DEFINITION NOT FOUND <a class="footnote-return" href="#fnref:4"><sup>↩</sup></a></li>
        </ol>
        </div>

         wants:
          <p>Test 1<sup class="footnote-ref" id="fnref:1"><a rel="footnote" href="#fn:1">1</a></sup> and Test 2[fn: 2] and Test 3<sup class="footnote-ref" id="fnref:3"><a rel="footnote" href="#fn:3">2</a></sup> also test lettres<sup class="footnote-ref" id="fnref:let"><a rel="footnote" href="#fn:let">3</a></sup> then final test<sup class="footnote-ref" id="fnref:4"><a rel="footnote" href="#fn:4">4</a></sup></p>

        <h1 id="footnotes">Footnotes</h1>
        <div class="footnotes">

        <hr />

        <ol>
        <li id="fn:1">Test 1 <a class="footnote-return" href="#fnref:1"><sup>↩</sup></a></li>

        <li id="fn:3">Test 3 <a class="footnote-return" href="#fnref:3"><sup>↩</sup></a></li>

        <li id="fn:let">what? <a class="footnote-return" href="#fnref:let"><sup>↩</sup></a></li>

        <li id="fn:4">DEFINITION NOT FOUND <a class="footnote-return" href="#fnref:4"><sup>↩</sup></a></li>
        </ol>
        </div>
    goorgeous_test.go:431: multiline: Footnote for Org() from: 
         Test 1[fn:1]

        * Footnotes

        [fn:1] Test 1
        More details 
         result: 
          <p>Test 1<sup class="footnote-ref" id="fnref:1"><a href="#fn:1">1</a></sup></p>

        <h1 id="footnotes">Footnotes</h1>
        <div class="footnotes">

        <hr />

        <ol>
        <li id="fn:1">Test 1 More details <a class="footnote-return" href="#fnref:1"><sup>↩</sup></a></li>
        </ol>
        </div>

         wants:
          <p>Test 1<sup class="footnote-ref" id="fnref:1"><a rel="footnote" href="#fn:1">1</a></sup></p>

        <h1 id="footnotes">Footnotes</h1>
        <div class="footnotes">

        <hr />

        <ol>
        <li id="fn:1">Test 1 More details <a class="footnote-return" href="#fnref:1"><sup>↩</sup></a></li>
        </ol>
        </div>

Many thanks!