selftests/bpf: Fix memory leak caused by not destroying skeleton

Some test cases does not destroy skeleton object correctly, causing ASAN
to report memory leak warning. Fix it.

Fixes: 0ef6740e9777 ("selftests/bpf: Add tests for kptr_ref refcounting")
Fixes: 1642a3945e22 ("selftests/bpf: Add struct argument tests with fentry/fexit programs.")
Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/bpf/20221011120108.782373-4-xukuohai@huaweicloud.com
