Add barrier to access chain workgroup tests

Add barrier to access chain workgroup tests

These tests can complete with a failure to match expected
result in the last array element due to a race condition
with the parallel work-items.

The first work-item should set the last array element to 0
before a function to read the last array element is executed
on a parallel work-item.

Without the barrier it's possible on occasional test runs
to read uninitialised memory into the last array element.

Components: Vulkan

VK-GL-CTS issue: 1893

Affects:
dEQP-VK.spirv_assembly.instruction.compute.ptr_access_chain.*

Change-Id: I072299f7995b8f65e26195cfddd12f49cffcc9ff