Shaders Derivate Tests: fix threshold

Shaders Derivate Tests: fix threshold

Derivate tests are calculating the threshold used for validating results
by taking in account a scale. Apparently this is incorrect, and is
causing some tests to fail.

A similar situation happened in Vulkan, and the proper fix was to do not
take in account the scale for the threshold calculation. Hence, applying
the same fix here fixes all the failing tests.

Affects:
dEQP-GLES3.functional.shaders.derivate.*
dEQP-GLES31.functional.shaders.derivate.*

Components: OpenGL

VK-GL-CTS issue: 1237

Change-Id: I106e36a708a6f75768c15ccdb4b064eb562cb28f