selftests/tracing: Add basic test for trace_marker_raw file

selftests/tracing: Add basic test for trace_marker_raw file

Commit 64cf7d058a00 ("tracing: Have trace_marker use per-cpu data to read
user space") made an update that fixed both trace_marker and
trace_marker_raw. But the small difference made to trace_marker_raw had a
blatant bug in it that any basic testing would have uncovered.
Unfortunately, the self tests have tests for trace_marker but nothing for
trace_marker_raw which allowed the bug to get upstream.

Add basic selftests to test trace_marker_raw so that this doesn't happen
again.

Link: https://lore.kernel.org/r/20251014145149.3e3c1033@gandalf.local.home
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>