Merge branch '392-split-tests' into 'vulkan-cts-1.0-dev'

Fix issues with undefined levels in tess primitive discard tests

In certain cases less than 1 inner tessellation levels result in
implementation-dependent number of vertices to be generated. Relax
verification in those cases, and add separate tests that exercise only
valid (>= 1) inner levels.

Fixes #392

See merge request !592