GL_ARB_enhanced_layouts: no def in untested stage

GL_ARB_enhanced_layouts: no def in untested stage

Currently we are trying to match the definition of the output block
with the input block. In a negative test this has the problem that we
cannot actually match without adding the same error we are trying to
test in both interfaces. Therefore, as it is right now, even if the
tested error wouldn't happen, a misleading linking error will happen.

We avoid this by removing the input definition when testing
outputs (no need to have them at all) and, in the reverse situation,
additionally, by not doing static use of the defined input variable.

Additionally, this simplifies the test a lot.

Finally, added a debugging macro to ease checking that we are testing
just the correct and only cause for a failure report.

Components: OpenGL

VK-GL-CTS issue: 1602

Affects:

KHR-GL44.enhanced_layouts.varying_block_member_locations

Change-Id: Ia2c0ba705e975817999aaa16c1ce129d616540f6