selftests/bpf: cover UTF-8 trace_printk output

selftests/bpf: cover UTF-8 trace_printk output

Extend trace_printk coverage to verify that UTF-8 literal text is
emitted successfully and that '%' parsing still rejects non-ASCII
bytes once format parsing starts.

Use an explicitly invalid format string for the negative case so the
ASCII-only parser expectation is visible from the test code itself.

Signed-off-by: Yihan Ding <dingyihan@uniontech.com>
Acked-by: Paul Chaignon <paul.chaignon@gmail.com>
Link: https://lore.kernel.org/r/20260416120142.1420646-3-dingyihan@uniontech.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>