[cmake] Check for deps before adding.

[cmake] Check for deps before adding.

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/+/26788
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Dan Sinclair <dsinclair@google.com>