selftests/bpf: Add missing cleanup in atomic_bounds test

Add missing skeleton destroy call.

Fixes: 37086bfd ("bpf: Propagate stack bounds to registers in atomics w/ BPF_FETCH")
Reported-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210208123737.963172-1-jackmanb@google.com