selftests/bpf: Add selftests verifying bpf_trace_printk() behaviour

Simple selftests that verifies bpf_trace_printk() returns a sensible
value and tracing messages appear.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/1594641154-18897-3-git-send-email-alan.maguire@oracle.com