selftests: bpf: use __stderr in stream error tests

Start using __stderr directly in the bpf programs to test the reporting
of may_goto timeout detection and spin_lock dead lock detection.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250911145808.58042-6-puranjay@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>