selftests/tracing: Fix test_multiple_writes stall

selftests/tracing: Fix test_multiple_writes stall

When /sys/kernel/tracing/buffer_size_kb is less than 12KB,
the test_multiple_writes test will stall and wait for more
input due to insufficient buffer space.

Check current buffer_size_kb value before the test. If it is
less than 12KB, it temporarily increase the buffer to 12KB,
and restore the original value after the tests are completed.

Link: https://lore.kernel.org/r/20260109033620.25727-1-fushuai.wang@linux.dev
Fixes: 37f46601383a ("selftests/tracing: Add basic test for trace_marker_raw file")
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Fushuai Wang <wangfushuai@baidu.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>