Require VK_KHR_shader_float16_int8 in float16 opconstant tests

Require VK_KHR_shader_float16_int8 in float16 opconstant tests

The SPIR-V code in the shaders used by these tests includes
16-bit floating point OpVectorTimesScalar and
OpMatrixTimesVector operations, which require the extension. There
is no compute version of these tests, only graphics.

Components: Vulkan
VK-GL-CTS issue: 1403

Affects:
dEQP-VK.spirv_assembly.instruction.graphics.float16.opconstant.*

Change-Id: If0f7960013759a0d5dad41629dcefc79350c3c88
(cherry picked from commit a83c04cd2f258fbce81ddbaa10f1e5ae092deb8b)