selftests/bpf: Ensure consistent test failure output

printf() doesn't seem to honor using overwritten stdout/stderr (as part of
stdio hijacking), so ensure all "standard" invocations of printf() do
fprintf(stdout, ...) instead.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200313172336.1879637-2-andriin@fb.com