Test readonly and writeonly buffer variables

Test readonly and writeonly buffer variables

It is allowed the both readonly and writeonly qualifiers be used on
buffer variables. It just means it can't be read or written, but can
otherwise be referenced like with .size(), etc.

Components: OpenGL

VK-GL-CTS issue: 639

Affects:
KHR-GL43.shader_storage_buffer_object.basic-readonly-writeonly
KHR-GLES31.core.shader_storage_buffer_object.basic-readonly-writeonly

Change-Id: Id8f68a8687e41cdd42d45c8300f812833312517e