selftests: bpf: introduce __stderr and __stdout

selftests: bpf: introduce __stderr and __stdout

Add __stderr and __stdout to validate the output of BPF streams for bpf
selftests. Similar to __xlated, __jited, etc., __stderr/out can be used
in the BPF progs to compare a string (regex supported) to the output in
the bpf streams.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250911145808.58042-5-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>