You don't necessarily need a flying engine testbed, there have been several engines developed that didn't have a testbed aircraft and were directly integrated into prototypes for testing, like the F135 engine for the F-35, never got tested on a dedicated flying testbed. Similarly the Original engine for the Gripen, the RM12 was tested directly on the gripen prototype. Same with the Snecma M88 on the Rafale.
The dedicated flying testbed is nice to have, but not an absolute necessity. They allow for saving time not needing as much ground testing with the initial prototype aircraft and for safety reasons, but again, not a necessity, especially with twin engined aircraft, which you fly with one engine being the initial engine and the second being the test engine, the Al-51 on the Su-57 was tested in this manor, despite Russia having an Il-76 testbed, b/c there are unique advantages to testing the prototype engine directly on the intended aircraft early on.
Lastly, you don't think Turkey can acquire a testbed aircraft? lol, TEI has had a relationship with GE, I literally mentioned in this very same thread that Turkey license produces GE engines, you aren't exactly breaking any news by pointing out the relationship with GE, I literally mentioned it as a point of strength for Turkey. And GE possesses a Boeing 777 testbed. Turkey also has a defense relationship with the UK and RollsRoyce, which initially offered its help in codeveloping this very same engine.
Put simply, Turkey has many options with regards to how it goes about developing the engine on whatever testbed and whatever manor it wishes. lol