selftests/bpf: Add checks for X__elf_bytes() skeleton helper

This patch adds two checks for the X__elf_bytes BPF skeleton helper
method. The first asserts that the pointer returned from the helper
method is valid, the second asserts that the provided size pointer is
set.
Signed-off-by: Matt Smith <alastorze@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210901194439.3853238-4-alastorze@fb.com

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

@@ -18,6 +18,8 @@ void test_skeleton(void)
 	struct test_skeleton__data *data;
 	struct test_skeleton__rodata *rodata;
 	struct test_skeleton__kconfig *kcfg;
+	const void *elf_bytes;
+	size_t elf_bytes_sz = 0;
 
 	skel = test_skeleton__open();
 	if (CHECK(!skel, "skel_open", "failed to open skeleton\n"))
@@ -91,6 +93,10 @@ void test_skeleton(void)
 	CHECK(bss->kern_ver != kcfg->LINUX_KERNEL_VERSION, "ext2",
 	      "got %d != exp %d\n", bss->kern_ver, kcfg->LINUX_KERNEL_VERSION);
 
+	elf_bytes = test_skeleton__elf_bytes(&elf_bytes_sz);
+	ASSERT_OK_PTR(elf_bytes, "elf_bytes");
+	ASSERT_GE(elf_bytes_sz, 0, "elf_bytes_sz");
+
 cleanup:
 	test_skeleton__destroy(skel);
 }