Remove need for `TEST_*` define in tests

[alexreinking]

Currently, seven tests use a macro to determine whether certain GPU libraries are available. This information should be available at runtime to avoid special-casing in build.

The tests are:

• generator/opencl_runtime_aottest.cpp (opencl)
  • This one doesn't actually use its generator, it just needs it for the runtime.
• generator/acquire_release_aottest.cpp (opencl, cuda)
• generator/buffer_copy_aottest.cpp (opencl, cuda)
• generator/cxx_mangling_aottest.cpp (cuda)
• generator/define_extern_opencl_aottest.cpp (opencl)
• generator/gpu_object_lifetime_aottest.cpp (opencl, cuda, metal)
• generator/gpu_only_aottest.cpp (opencl, cuda)