selftests/bpf: Use explicit bpf_prog_test_load() calls everywhere

-Dbpf_prog_load_deprecated=bpf_prog_test_load trick is both ugly and
breaks when deprecation goes into effect due to macro magic. Convert all
the uses to explicit bpf_prog_test_load() calls which avoid deprecation
errors and makes everything less magical.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/bpf/20211103220845.2676888-12-andrii@kernel.org

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

@@ -20,7 +20,7 @@ void test_skb_helpers(void)
 	struct bpf_object *obj;
 	int err;
 
-	err = bpf_prog_load("./test_skb_helpers.o", BPF_PROG_TYPE_SCHED_CLS, &obj,
+	err = bpf_prog_test_load("./test_skb_helpers.o", BPF_PROG_TYPE_SCHED_CLS, &obj,
 			    &tattr.prog_fd);
 	if (CHECK_ATTR(err, "load", "err %d errno %d\n", err, errno))
 		return;