Windows DLLs are not systematically prefixed with "lib".
This change follows the implicit and explict expectations set out in:
https://vulkan.lunarg.com/doc/view/1.0.21.0/windows/LoaderAndLayerInterface.html
http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html#AEN49

Also make the vk_swiftshader target a dependency of vk-unittests so that
building the latter also allows running it.

Bug: b/139189696
Change-Id: I070871e63c5cefc3f80a6f441a1d5097c85b82b6
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/35630
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>