Did a quick search and couldn't find anything on this so here we go. Sorry if this is indeed a repost.
As you probably know on the ptr if you enter the old world on a flying mount (that is in flight mode not ground mound) via a portal in dalaran you can fly in the old world until you leave the sub zone you are in. Once you leave that sub zone you are dismounted. However with flight form you can stay in the form and not get dismounted and thus can fly anywhere.
To recreate: Go to Krasus' Landing in Dalaran, activate flight form/swift flight form then proceed to fly across dalaran to your faction's portals and quickly enter the one where you wish to fly. That's all there is to it, enjoy.
Warning: Some places may dismount you, I have only found one such place and that was flying back from azshara into northern orgrimmar, corpse landed on a noob ><.