Allow tests to pass with a ES3+ context.

Allow tests to pass with a ES3+ context.

Setting gl_MaxDrawBuffers ref value to GL_MAX_DRAW_BUFFERS for ES3 context.

Reduce code duplication where the same check exists in other tests.

Components: OpenGL ES

VK-GL-CTS issue: 2011

Affected tests:
dEQP-GLES2.functional.shaders.builtin_variable.max_draw_buffers_vertex
dEQP-GLES2.functional.shaders.builtin_variable.max_draw_buffers_fragment
dEQP-GLES2.functional.negative_api.buffer.framebuffer_texture2d
dEQP-GLES2.functional.texture.mipmap.cube.*

Change-Id: I6c5e522940256c98b54e5533f29fdafeac60a256