[Piglit] Core vs compat 3.1 contexts

[Ilia Mirkin]

All the arb_shader_atomic_counters tests start with

        config.supports_gl_core_version = 31;

However when I try to run them on the NVIDIA proprietary driver, I get:

piglit: info: Requested a OpenGL 3.1 Context, but the actual context
is a 3.1 context that exposes the GL_ARB_compatibility extension
piglit: info: Failed to create any GL context
PIGLIT: {'result': 'skip' }

This seems to come from piglit_wfl_framework.c:special_case_gl_31.

Unfortunately I know ~nothing about the subtleties of all this, but
what's the way forward? Is the driver at fault? Piglit? FTR, changing
the supports_gl_core_version to 32 fixes it.

  -ilia