Test descriptor_update_template, push_descriptor

Test descriptor_update_template, push_descriptor

Extended the existing binding_model tests with 3 new variants,
'with_template', 'with_push_template' and 'with_push'.

The least invasive method to support push descriptor updates without
template was to persist the DescriptorSetUpdateBuilder object in the
test instance classes, so that it can be referenced when building the
command buffer.

Change-Id: I731fa41c5ff99ab1d390d77d7aabb4424e091e35
(cherry picked from commit 7f5ba3914c1d9033dd4f896ef6fcaa22f5c5e038)