gtest: extend testing up to 8 GPUs

test/test_Reduce.cpp

@@ -63,7 +63,7 @@ namespace CorrectnessTests
                                 // Number of elements
                                 testing::Values(1024, 1048576),
                                 // Number of devices
-                                testing::Values(2,3,4),
+                                testing::Values(2,3,4,5,6,7,8),
                                 // In-place or not
                                 testing::Values(false, true),
                                 testing::Values("")));