COMP: Reduce memory usage for TestCheckerboard

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

Change-Id: I074012d39f017f03463c240e96635820c8abdbae
Merge topic 'reduce-tcl_test-checkboard-memory-usage' into master

c250dadc COMP: Reduce memory usage for TestCheckerboard