selftests/bpf: Add tests for special fields races

Add a couple of tests to ensure that the refcount drops to zero when we
exercise the race where creation of a special field succeeds the logical
bpf_obj_free_fields done when deleting an element. Prior to previous
changes, the fields would be freed eagerly and repopulate and end up
leaking, causing the reference to not drop down correctly. Running this
test on a kernel without fixes will cause a hang in delete_module, since
the module reference stays active due to the leaked kptr not dropping
it. After the fixes tests succeed as expected.

Reviewed-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260227224806.646888-6-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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