selftests/bpf: Fix bpf_object leak in skb_ctx selftest

skb_ctx selftest didn't close bpf_object implicitly allocated by
bpf_prog_test_load() helper. Fix the problem by explicitly calling
bpf_object__close() at the end of the test.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Hengqi Chen <hengqi.chen@gmail.com>
Link: https://lore.kernel.org/bpf/20211107165521.9240-10-andrii@kernel.org

tools/testing/selftests/bpf/prog_tests/skb_ctx.c

@@ -111,4 +111,6 @@ void test_skb_ctx(void)
 		   "ctx_out_mark",
 		   "skb->mark == %u, expected %d\n",
 		   skb.mark, 10);
+
+	bpf_object__close(obj);
 }