Reduce memory usage for TestCheckerboard

This reduces the memory usage for TestCheckerboard by reducing the
Z extents of the images from 99 to 10. This reduces the memory usage
of the test from ~170MB to ~37MB.

This should fix the test on AIX where it was running out of resources.

Change-Id: I3513406e35a88793eff96d36f07c8497e15401f4
Merge topic 'reduce-test-checkboard-memory-usage' into master

96ae86c7 Reduce memory usage for TestCheckerboard