Add simulated error before rounding to target precision in dfdx tests.
Add simulated error before rounding to target precision in dfdx tests.

- Add interpolation error before rounding in GLES3 Derivate tests. If the
  error is not added before rounding and reference value happens to be 1:1
  representable in the target precision, both rounding directions yield the
  same value and error bounds will be lost.

Bug: 21666909
Change-Id: Ie948ea32075439c962d8ca52362069ca030a172e

KhronosGroup/VK-GL-CTS,chadversary/deqp,chadversary/deqp,KhronosGroup/VK-GL-CTS,geekboxzone/mmallow_external_deqp,KhronosGroup/VK-GL-CTS,chadversary/deqp,googlestadia/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,geekboxzone/mmallow_external_deqp,geekboxzone/mmallow_external_deqp,KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,chadversary/deqp,googlestadia/VK-GL-CTS,chadversary/deqp,KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,geekboxzone/mmallow_external_deqp,KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,geekboxzone/mmallow_external_deqp,chadversary/deqp,geekboxzone/mmallow_external_deqp,googlestadia/VK-GL-CTS