Return a valid format when GL_RGB8 is not treated as GL_RGBA8

Return a valid format when GL_RGB8 is not treated as GL_RGBA8

There was a special case added to NearestEdgeTestCase::toTextureFormat()
in https://gerrit.khronos.org/c/vk-gl-cts/+/6634 to support implementations
that treat GLRGB8 internally as GL_RGBA8888. This code change had a coding
error is that for implementations that don't support GLRGB8 in this way
ended up aborting with a fatal error:
    "Unable to map pixel format to texture format"

This trivial fix corrects the coding bug to simply return the
unadjusted format.

Affects:

KHR-GLES3.core.nearest_edge.offset_*
KHR-GLES31.core.nearest_edge.offset_*

Components: OpenGL

VK-GL-CTS issue: 2758

Change-Id: I639aed8bfed70941688d30edb02a770be4b05b08