selftests/bpf: Add checking of raw type dump in BTF writer APIs selftests

Add re-usable btf_helpers.{c,h} to provide BTF-related testing routines. Start
with adding a raw BTF dumping helpers.

Raw BTF dump is the most succinct and at the same time a very human-friendly
way to validate exact contents of BTF types. Cross-validate raw BTF dump and
writable BTF in a single selftest. Raw type dump checks also serve as a good
self-documentation.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20201105043402.2530976-7-andrii@kernel.org
diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 50e5b18f..c1708ff 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -386,7 +386,7 @@
 TRUNNER_BPF_PROGS_DIR := progs
 TRUNNER_EXTRA_SOURCES := test_progs.c cgroup_helpers.c trace_helpers.c	\
 			 network_helpers.c testing_helpers.c		\
-			 flow_dissector_load.h
+			 btf_helpers.c	flow_dissector_load.h
 TRUNNER_EXTRA_FILES := $(OUTPUT)/urandom_read				\
 		       $(wildcard progs/btf_dump_test_case_*.c)
 TRUNNER_BPF_BUILD_RULE := CLANG_BPF_BUILD_RULE