selftests/bpf: Switch SEC("classifier*") usage to a strict SEC("tc")

Convert all SEC("classifier*") uses to a new and strict SEC("tc")
section name. In reference_tracking selftests switch from ambiguous
searching by program title (section name) to non-ambiguous searching by
name in some selftests, getting closer to completely removing
bpf_object__find_program_by_title().

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210928161946.2512801-4-andrii@kernel.org

tools/testing/selftests/bpf/progs/test_global_func7.c

@@ -10,7 +10,7 @@ void foo(struct __sk_buff *skb)
 	skb->tc_index = 0;
 }
 
-SEC("classifier/test")
+SEC("tc")
 int test_cls(struct __sk_buff *skb)
 {
 	foo(skb);