selftests/bpf: Test btf__load_vmlinux_btf/btf__load_module_btf APIs

Add test for btf__load_vmlinux_btf/btf__load_module_btf APIs. The test
loads bpf_testmod module BTF and check existence of a symbol which is
known to exist.
Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210815081035.205879-1-hengqi.chen@gmail.com

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

+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright (c) 2021 Hengqi Chen */
+
+#include <test_progs.h>
+#include <bpf/btf.h>
+
+static const char *module_name = "bpf_testmod";
+static const char *symbol_name = "bpf_testmod_test_read";
+
+void test_btf_module()
+{
+	struct btf *vmlinux_btf, *module_btf;
+	__s32 type_id;
+
+	if (!env.has_testmod) {
+		test__skip();
+		return;
+	}
+
+	vmlinux_btf = btf__load_vmlinux_btf();
+	if (!ASSERT_OK_PTR(vmlinux_btf, "could not load vmlinux BTF"))
+		return;
+
+	module_btf = btf__load_module_btf(module_name, vmlinux_btf);
+	if (!ASSERT_OK_PTR(module_btf, "could not load module BTF"))
+		goto cleanup;
+
+	type_id = btf__find_by_name(module_btf, symbol_name);
+	ASSERT_GT(type_id, 0, "func not found");
+
+cleanup:
+	btf__free(module_btf);
+	btf__free(vmlinux_btf);
+}