[IMPROV] tests: Document usage of write env vars
[IMPROV] tests: Document usage of write env vars

Instead of being specific to Travis make clear that also failure tests are
disabled be default and only enabled on Travis (except for the 'wikimedia'
user). It also documents that both variables work independently so that failure
tests aren't done when normal tests are enabled.

Change-Id: I47561267cb905fcf84fd986ef9dd15a85cf20b10

Darkdadaah/pywikibot-core,Darkdadaah/pywikibot-core,h4ck3rm1k3/pywikibot-core,wikimedia/pywikibot-core,happy5214/pywikibot-core,npdoty/pywikibot,smalyshev/pywikibot-core,valhallasw/pywikibot-core,wikimedia/pywikibot-core,xZise/pywikibot-core,TridevGuha/pywikibot-core,jayvdb/pywikibot-core,hasteur/g13bot_tools_new,npdoty/pywikibot,icyflame/batman,h4ck3rm1k3/pywikibot-core,VcamX/pywikibot-core,happy5214/pywikibot-core,PersianWikipedia/pywikibot-core,darthbhyrava/pywikibot-local,magul/pywikibot-core,magul/pywikibot-core,trishnaguha/pywikibot-core,hasteur/g13bot_tools_new,hasteur/g13bot_tools_new,jayvdb/pywikibot-core,emijrp/pywikibot-core