This CL refactors most of the existing tests for the
GL_EXT_shader_framebuffer_fetch_non_coherent extension, sharing shader
and test code with new tests for the coherent extension.

Bug: b/188095445
Change-Id: I23eac5dae9055f1c8f9eeb9c33429fd1091e68fb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2923323Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Ian Elliott <ianelliott@google.com>