framework: Add a vulkan tests profile

This searches for files named *.vk_shader_test in the tests/vulkan
directory and runs them with VkRunner. VkRunner is executed as an
external dependency. It is found either with the vkrunner:bin config
option, by setting the PIGLIT_VKRUNNER_BINARY environment variable, or
just in the search path.

v2: Move VkShaderTest to piglit_test.py and rename to VkRunnerTest.
    Add future imports. Remove unused import.
v3: Support the PIGLIT_VKRUNNER_BINARY variable to specify the
    location of VkRunner.
v4: Add documentation to the README. Add an option in piglit.conf to
    set the binary location. (Suggested by Samuel Iglesias)

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>