This commit refactors the `add_offload_test_device_code` CMake function
to compile device code using the C++ compiler (`CMAKE_CXX_COMPILER`)
instead of the C compiler.

This change enables the use of C++ features, such as templates, within
device-side test kernels. This will allow for more advanced and reusable
kernel wrappers, reducing boilerplate code in the conformance test
suite.

As part of this change:
- All `.c` files for device code in `unittests/` have been renamed to
`.cpp`.
- Kernel definitions are now wrapped in `extern "C"` to ensure C linkage
and prevent name mangling.

This change affects the `OffloadAPI` and `Conformance` test suites.

cc @callumfare @RossBrunton @jhuber6