Add CMAKE_TEST_LAUNCHER capability and gpu labels

[wdeconinck]

Before this PR, the serial tests that require a GPU resource would fail when using

salloc -q dg -n 4 --gpus-per-task=1 ctest

The issue is those tests need to be launched with srun -n 1 within ctest to gain access to the GPU resource.

Adding the CMAKE_TEST_LAUNCHER (available since cmake 3.29) allows to run serial tests to be run with a launcher, in this case srun -n 1.
This feature used to be already available in versions of cmake < 3.29 under the more obscure name CMAKE_CROSSCOMPILING_EMULATOR. This PR makes the use of CMAKE_TEST_LAUNCHER forward compatible.

I also added labels "gpu" to GPU tests.

Following workflow works now on our HPC:

Before configuration:

export CMAKE_TEST_LAUNCHER="srun;-n;1"

To launch only gpu tests:

salloc -q dg -n 4 --gpus-per-task=1 ctest -L gpu

[samhatfield]

Nice work, very neat and tidy. I tested with the following CMakes on ECMWF/AC:

• cmake/3.28.3
• cmake/3.25.2

[marsdeno]

Looks good to me