vktRayTracingMiscTests.cpp: abs -> fabsf

vktRayTracingMiscTests.cpp: abs -> fabsf

Fixes these errors building for FreeBSD 10.4 with llvm60:
/root/nvt/tests/buildvulkancts/vk-gl-cts/external/vulkancts/modules/vulkan/ray_tracing/vktRayTracingMiscTests.cpp:2248:11: error: call to 'abs' is ambiguous
                                                if (abs(currentItem.dataChunk.at(nItem) - static_cast<float>(nRay + nItem)) > 1e-3f)
                                                    ^~~
/usr/include/stdlib.h:83:6: note: candidate function
int      abs(int) __pure2;
         ^
/usr/include/c++/v1/cstdlib:159:44: note: candidate function
inline _LIBCPP_INLINE_VISIBILITY long      abs(     long __x) _NOEXCEPT {return  labs(__x);}
                                           ^
/usr/include/c++/v1/cstdlib:161:44: note: candidate function
inline _LIBCPP_INLINE_VISIBILITY long long abs(long long __x) _NOEXCEPT {return llabs(__x);}
                                           ^

VK-GL-CTS issue: 2696

Components: Vulkan

Affects:
dEQP-VK.ray_tracing_pipeline.misc.callableshaderstress*

Change-Id: I0be12116f368d02251a7b1d6a560d1ff898f78dd