selftests: bpf: Move bpf_printk to bpf_helpers.h

bpf_printk is a macro which is commonly used to print out debug messages
in BPF programs and it was copied in many selftests and samples. Since
all of them include bpf_helpers.h, this change moves the macro there.

Signed-off-by: Michal Rostecki <mrostecki@opensuse.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>