While the Mage Tower itself has not been added for testing on the 9.1.5 PTR, these new achievements indicate that we may get to see it soon.
There is also the chance that to keep the Mage Tower Challenging on release, Blizzard may choose to not have it publically tested on the PTR.
Three New Achievements For Completing The Legion Timewalking Mage Tower
A Towering Success - Complete a challenge for any specialization at the Mage Tower during Legion Timewalking.
A Tour of Towers - Complete every unique challenge available at the Mage Tower during Legion Timewalking
Tower Overwhelming - Completed every challenge at the Mage Tower across all classes and specializations during the Legion Timewalking event.
Blizzard confirmed that the Soaring Spelltome mount will be rewarded upon clearing all unique challenges and will likely be awarded as part of the A Tour of Towers achievement.
The new achievements have not come without controversy, however, as players who completed all 36 unique challenges in Legion were not rewarded with achievements such as Tower Overwhelming but now there is one for the Timewlaking event.
Many players wish to have the old Mage Tower skins as rewards, but as they were tied behind the removed version of the Mage Tower, they will not return and will stay unobtainable, and introducing an achievement for players who completed them all during Legion may see as them favoring one side of the argument so players are unlikely to see a new achievement for Legion players.