Clear the stencil attachment if it exists

Clear the stencil attachment if it exists

The test uses VK_STENCIL_OP_INCREMENT_AND_WRAP which needs the
stencil attachment in a known state in order to produce
predictable results.

Affects:

dEQP-VK.renderpass.suballocation.multisample.s8_uint.samples_*
dEQP-VK.renderpass.suballocation.multisample.d16_unorm_s8_uint.samples_*
dEQP-VK.renderpass.suballocation.multisample.d24_unorm_s8_uint.samples_*
dEQP-VK.renderpass.suballocation.multisample.d32_sfloat_s8_uint.samples_*
dEQP-VK.renderpass.multisample.s8_uint.samples_*
dEQP-VK.renderpass.multisample.d16_unorm_s8_uint.samples_*
dEQP-VK.renderpass.multisample.d24_unorm_s8_uint.samples_*
dEQP-VK.renderpass.multisample.d32_sfloat_s8_uint.samples_*

Components: Vulkan

VK-GL-CTS issue: 584

Change-Id: I7ed5ced8b77a4563301320e50b2c247e95c34936