Relax XFB begin check to match recent spec edit

Relax XFB begin check to match recent spec edit

Update tests to reflect a recent change to the GL 4.6 specification.
Previously, if a shader layout qualifier specified that xfb_stride was
non-zero for a given xfb_buffer, then a buffer object was required to
be bound at that transform feedback buffer binding point. After the
spec change, it is now legal to not have a buffer bound, provided no
outputs are associated with the buffer.

Components: OpenGL

VK-GL-CTS issue: 785

Affects:
* KHR-GL44/45/46.enhanced_layouts.xfb_stride_of_empty_list
* KHR-GL44/45/46.enhanced_layouts.xfb_stride_of_empty_list_and_api

Change-Id: I38622bad1af3bc8a2a4c683eadc184e6231f7c85