Refactor vktShaderExecutor

Refactor vktShaderExecutor

ShaderExecutor Vulkan port had several issues:

 * Lifetime of ShaderExecutor was designed to extend from TestCase to
TestInstance. This was done in order to allow generating shader sources at
TestCase and later reuse same object to do actual execution when Vulkan
device was available. Result was total lifetime mess and leak of Vulkan
objects beyond TestInstance lifetime.

 * Whole UniformSetup system was badly designed mess that provided
functionality for just a few tests.

Following cleanups and fixes have been made:

 * ShaderExecutor objects are now expected to be live only when Vulkan
device is available, i.e. only within vkt::TestInstance. Shader source
generation part has been moved into a separate utility.

 * UniformSetup mess is removed and replaced with much leaner and
extensible way to supply additional resources. Descriptor set 1 is
reserved for test case specific resources and those can be provided by
supplying custom descriptor layout and descriptor set. Tests using old
UniformSetup system have been adjusted accordingly.

There is still room for additional improvements, such as moving much of
pipeline construction from ShaderExecutor::execute() into constructors,
but these can be done in follow-up CLs.

Affects following test groups:

dEQP-VK.glsl.builtin.*
dEQP-VK.glsl.opaque_type_indexing.
dEQP-VK.texture.explicit_lod.*

VK-GL-CTS issue: 182
Components: Vulkan

Change-Id: Icfc45a36cb4651f39f12608842ef347a55353d84