bpf: Add test ops for BPF_PROG_TYPE_TRACING

The current fexit and fentry tests rely on a different program to
exercise the functions they attach to. Instead of doing this, implement
the test operations for tracing which will also be used for
BPF_MODIFY_RETURN in a subsequent patch.

Also, clean up the fexit test to use the generated skeleton.

Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200304191853.1529-7-kpsingh@chromium.org

kernel/trace/bpf_trace.c

@@ -1266,6 +1266,7 @@ const struct bpf_verifier_ops tracing_verifier_ops = {
 };
 
 const struct bpf_prog_ops tracing_prog_ops = {
+	.test_run = bpf_prog_test_run_tracing,
 };
 
 static bool raw_tp_writable_prog_is_valid_access(int off, int size,