Allow RTZ rounding in spv_assembly*writes_two_buffers* tests

Allow RTZ rounding in spv_assembly*writes_two_buffers* tests

Because the tests perform an add operation, mismatch of the CPU and GPU
rounding modes can cause the tests to incorrectly fail.
This change modifies the verification to allow up to 1 ULP difference
between the output and the expected result, which covers the maximum
difference between RNE and RTZ rounding modes.

Components: Vulkan

Affects: dEQP-VK.spirv_assembly.instruction.graphics*

VK-GL-CTS issue: 2320

Change-Id: Ic76942add1382c23885118f806ac593cc730eb73