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_pkt_md_access.c

@@ -7,8 +7,6 @@
 #include <linux/pkt_cls.h>
 #include <bpf/bpf_helpers.h>
 
-int _version SEC("version") = 1;
-
 #if  __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
 #define TEST_FIELD(TYPE, FIELD, MASK)					\
 	{								\
@@ -27,7 +25,7 @@ int _version SEC("version") = 1;
 	}
 #endif
 
-SEC("classifier/test_pkt_md_access")
+SEC("tc")
 int test_pkt_md_access(struct __sk_buff *skb)
 {
 	TEST_FIELD(__u8,  len, 0xFF);