tests: avoid ERR_TOO_MANY_REDIRECTS on frontend tests under Windows
tests: avoid ERR_TOO_MANY_REDIRECTS on frontend tests under Windows

If Etherpad is hosted on Windows the frontend test URI needs to be
/tests/frontend/index.html (docs say .../frontend/), otherwise there is this
error: ERR_TOO_MANY_REDIRECTS.

Fixes #3804.

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