The 2 second sleep can cause the test to fail on very slow network file
systems because Perf ends up being killed before it finishes starting
up.

Fix it by making the leafloop workload end after a fixed time like the
other workloads so there is no need to kill it after 2 seconds.

Also remove the 1 second start sampling delay because it is similarly
fragile. Instead, search through all samples for a matching one, rather
than just checking the first sample and hoping it's in the right place.

Fixes: cd6382d8 ("perf test arm64: Test unwinding using fame-pointer (fp) mode")
Signed-off-by: James Clark <james.clark@arm.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: German Gomez <german.gomez@arm.com>
Cc: Spoorthy S <spoorts2@in.ibm.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240612140316.3006660-1-james.clark@arm.com