Steps to reproduce:
1. Open attached project "SubstanceLeaking.zip"
2. Open "Playground" scene
3. Open profiler and observe the memory tab or open Task manager and watch Unity's used memory
4. Select "Tester" gameobject and select "Initial Generation Concurrent" in the "Test to run" field
5. Observe memory pooling up and not being cleared even when play mode is stopped.

Expected: Memory should be cleaned up after Gameobject/texture is destroyed.
Actual: In a very short time period memory will pool up and not get cleaned up until you shut down the editor

Reproduced on: 5.6.5p2, 2017.1.3p1, 2017.2.2f1, 2017.3.1p2