Blizzard have announced upcoming Mythic+ testing on the 10.2 PTR. Starting on September 14th, players will be able to enter 6 dungeons until September 18th.
M+ Testing (Source)
Dragonflight Season 3 Mythic+ testing begins on PTR realms Thursday September 14th. The test period will begin Thursday September 14th at 10:00 PDT (13:00 EDT, 20:00 CEST), and end Monday September 18th at 10:00 PDT (13:00 EDT, 20:00 CEST). Please note that the test period may be adjusted in the event of technical difficulties.
This first weekend includes 6 of our season 3 Mythic+ dungeons featuring both Dawn of the Infinite dungeon wings and updates to the doors in Waycrest Manor:
• Galakrond's Fall • Murozdon's Rise • Waycrest Manor • Atal'Dazar • Darkheart Thicket • Black Rook Hold
During the test period you’ll be able to acquire and customize Mythic Keystones by talking to the Keystone Vendor in Valdrakken, and the nearby Dungeon Teleports NPC will assist you with transportation.
We look forward to hearing your feedback!