selftests/bpf: Reuse stderr parsing for libarena ASAN tests

selftests/bpf: Reuse stderr parsing for libarena ASAN tests

Add code to directly test the output of libarena ASAN tests.
The code reuses testing infrastructure originally for BPF streams
to verify that ASAN emits call stacks when the selftests trigger
a memory error.

Since stderr() testing uses logic from test_progs, it is only
available on the test_progs-based selftest runner. The standalone
runner still uses internal ASAN state to verify access errors are
triaged as expected.

Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260426190338.4615-9-emil@etsalapatis.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>