tests: Fix test_rst_to_html with docutils 0.17
tests: Fix test_rst_to_html with docutils 0.17

Freshly released docutils 0.17 produces a different HTML output
than its previous version so ensure to have a rst input that
produces the same HTML output with all docutils versions.

SoftwareHeritage/swh-web-ui,SoftwareHeritage/swh-web-ui,SoftwareHeritage/swh-web-ui