[libsycl][unit][CMake] Fix unittests gtest CMake target (#216344)
This was exposed by my recent change moving the `gtest` unit tests to be
called by `lit` (but still using `gtest`) but it seems the problem was
technically there before.
We need to use a special gtest target for the runtime build. Luckily
LLVM infra provides a cmake target we can depend on that will just make
it work. More info
[here](https://github.com/llvm/llvm-project/blob/ec26997e2e4606d97918a4a082c4f93ca38a6f46/third-party/unittest/CMakeLists.txt#L25)
if interested.
Other runtimes do the same thing, see
[here](https://github.com/llvm/llvm-project/blob/828d2d7fb65a9cd5946b347ad5173e3bdb21387d/openmp/tools/omptest/CMakeLists.txt#L88)
for example.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
llvm-project
9da823c8 - [libsycl][unit][CMake] Fix unittests gtest CMake target (#216344)
