selftests/tracing: Fix false failure of subsystem event test

The subsystem event test enables all "sched" events and makes sure there's
at least 3 different events in the output. It used to cat the entire trace
file to | wc -l, but on slow machines, that could last a very long time.
To solve that, it was changed to just read the first 100 lines of the
trace file. This can cause false failures as some events repeat so often,
that the 100 lines that are examined could possibly be of only one event.

Instead, create an awk script that looks for 3 different events and will
exit out after it finds them. This will find the 3 events the test looks
for (eventually if it works), and still exit out after the test is
satisfied and not cause slower machines to run forever.

Link: https://lore.kernel.org/r/20250721134212.53c3e140@batman.local.home


Reported-by: Tengda Wu <wutengda@huaweicloud.com>
Closes: https://lore.kernel.org/all/20250710130134.591066-1-wutengda@huaweicloud.com/


Fixes: 1a4ea83a ("selftests/ftrace: Limit length in subsystem-enable tests")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>