Multiple draw buffers in blend_equation_advanced

Multiple draw buffers in blend_equation_advanced

GL_NVX_blend_equation_advanced_multi_draw_buffers adds support for
multiple draw buffers with advanced blend equations. This change
modifies the below test to check for this extension and to expect
GL_NO_ERROR if it is present.

Components: AOSP

VK-GL-CTS issue: 770

Affects:
dEQP-GLES31.functional.debug.negative_coverage.get_error.advanced_blend.attachment_advanced_equation

(cherry picked from commit 2cb35a235c1a7bd3c63b296c37e186eec34e08e2)

Change-Id: I70ba495d96b4a17bed589e3affa693f572fde446