tests: in backend tests, use POST instead of GET for setText() and setHTML()
tests: in backend tests, use POST instead of GET for setText() and setHTML()

This is allowed starting from fc661ee13af1 ("core: allow URL parameters and POST
bodies to co-exist"), which landed in Etherpad 1.8.0. For the discussion, see
issue #3568.

joequant/etherpad-lite,joequant/etherpad-lite,storytouch/etherpad-lite,storytouch/etherpad-lite,ether/etherpad-lite,aptivate/etherpad-lite,storytouch/etherpad-lite,joequant/etherpad-lite,ether/etherpad-lite,aptivate/etherpad-lite,aptivate/etherpad-lite,aptivate/etherpad-lite,joequant/etherpad-lite,ether/etherpad-lite