[Bugfix]: Only run GPU tests on HIP backend

[daniellegillai]

Note: ran out of time to test that nothing broke on the HIP backend, and to review more carefully which tests should be HIP only. But with that test and review, this should otherwise be ready.

Motivation

Fixes #500: when rocAL is compiled with CPU backend, does not run the GPU tests.

Technical Details

Modified the tests/cpp_api/CMakeLists.txt to guard the GPU tests with a BACKEND check. Also emits a message when skipping GPU tests in cmake step.

Test Plan

Built TOT rpp and MIVisionX with CPU backends, then built rocAL with -DBACKEND=CPU. Ran CTests.
Note: ran out of time to test that nothing broke on the HIP backend, and to review more carefully which tests should be HIP only. But with that test and review, this should otherwise be ready.

Test Result

Only runs non-GPU tests on CPU backend, passes all:

Test project /home/danielle/rocAL/build-cpu
    Start 1: basic_test_cpu
1/8 Test #1: basic_test_cpu .....................   Passed    1.03 sec
    Start 2: dataloader_multithread_cpu
2/8 Test #2: dataloader_multithread_cpu .........   Passed    2.03 sec
    Start 3: performance_tests_cpu
3/8 Test #3: performance_tests_cpu ..............   Passed    1.01 sec
    Start 4: performance_tests_with_depth_cpu
4/8 Test #4: performance_tests_with_depth_cpu ...   Passed    1.09 sec
    Start 5: unit_tests_cpu
5/8 Test #5: unit_tests_cpu .....................   Passed    9.23 sec
    Start 6: serialization_test_cpu
6/8 Test #6: serialization_test_cpu .............   Passed    0.91 sec
    Start 7: checkpoint_save
7/8 Test #7: checkpoint_save ....................   Passed    1.78 sec
    Start 8: checkpoint_restore
8/8 Test #8: checkpoint_restore .................   Passed    2.01 sec

[kiritigowda]

@levxn can you validate this on both CPU and HIP backend.