perf test: Wait for a new thread when testing --per-thread record

Just running the target program is not enough to test multi-thread
target because it'd be racy perf vs target startup.  I used the
initial delay but it cannot guarantee for perf to see the thread.

Instead, use wait_for_threads helper from shell/lib/waiting.sh to make
sure it starts the sibling thread first.  Then perf record can use -p
option to profile the target process.

Reviewed-by: Adrian Hunter <[email protected]>
Signed-off-by: Namhyung Kim <[email protected]>
Acked-by: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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