When embedding SwitftShader into a third party application, the top level cmake files may already include googletest and SPIRV-Tools. This CL updates the SwiftShader tests to only try to pull in googletest if the `gtest` target is not found and only pull in SPIRV-Tools if the `SPIRV-Tools` target is not found. Change-Id: I3c3ebb528725017dbbb80aa05b01ee2de8d4a2d1 Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/26788Reviewed-by: Ben Clayton <bclayton@google.com> Reviewed-by: Nicolas Capens <nicolascapens@google.com> Tested-by: Dan Sinclair <dsinclair@google.com>
