CTS: Fix memory leak in RSBlendTest loop.

rscpp RSBlendTest does not call rs.destroy() and leaks memory for the RS
context at every loop iteration. Fix the leak by adding that call at end
of the test loop.

Fixes issue https://code.google.com/p/android/issues/detail?id=71157

.

Change-Id: I2910e784edf0b39de47cea4407a4d31d8e1aacf0
Signed-off-by: Kedar Patil <kepatil@nvidia.com>