tests: print expected & received output for 'getHTML'
tests: print expected & received output for 'getHTML'

This test was broken by #3268 (getLineHTMLForExport - Fixes #2486 but breaks
plugins). This change aims to facilitate debugging.

Expected:
<!doctype html><html><body><ul class="bullet"><li>one</li><li>2</li></ul><br><ul><ul class="bullet"><li>ul2</li></ul></ul></body></html>

Received:
<!doctype html><html><body><ul class="bullet"><li>one</li><li>2</ul><br><ul class="bullet"><li><ul class="bullet"><li>ul2</ul></li></ul></body></html>

cloudfoundry-community/etherpad-lite-cf,joequant/etherpad-lite,storytouch/etherpad-lite,storytouch/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,University-of-Potsdam-MM/etherpad-lite,aptivate/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,lpagliari/etherpad-lite,lpagliari/etherpad-lite,lpagliari/etherpad-lite,aptivate/etherpad-lite,aptivate/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,ether/etherpad-lite,storytouch/etherpad-lite,joequant/etherpad-lite,aptivate/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,lpagliari/etherpad-lite,ether/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,joequant/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,joequant/etherpad-lite,ether/etherpad-lite