Fix memory initialisation in subgroups tests

Fix memory initialisation in subgroups tests

Subgroup tests expand memory allocations to accommodate std140 layout
shaders; however, the extra memory is not initialised.
This changes initialisation to be based on the actually allocated
buffer size and adds additional verification to catch uninitialised
memory.

Affects:
dEQP-VK.subgroups.*

Components: Vulkan

VK-GL-CTS Issue: 1639

Change-Id: I0dfc0abb9bcbe63b5d1d5ad43e6fee99389fdfa2