perf test: Fix test_arm_coresight.sh failures on Juno

This test commonly fails on Arm Juno because the instruction interval
is large enough to miss generating any samples for Perf in system-wide
mode.

Fix this by lowering the interval until a comfortable number of Perf
instructions are generated. The test is still quick to run because only
a small amount of trace is gathered.

Before:

sudo ./perf test coresight -vvv
...
Recording trace with system wide mode
Looking at perf.data file for dumping branch samples:
Looking at perf.data file for reporting branch samples:
Looking at perf.data file for instruction samples:
CoreSight system wide testing: FAIL
...

After:

sudo ./perf test coresight -vvv
...
Recording trace with system wide mode
Looking at perf.data file for dumping branch samples:
Looking at perf.data file for reporting branch samples:
Looking at perf.data file for instruction samples:
CoreSight system wide testing: PASS
...

Reviewed-by: Leo Yan <[e-mail]>
Signed-off-by: James Clark <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Mathieu Poirier <[e-mail]>
Cc: Mike Leach <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Suzuki Poulouse <[e-mail]>
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/tests/shell/test_arm_coresight.sh

···
 	#   68.12%  touch    libc-2.27.so   [.] _dl_addr
 	#    5.80%  touch    libc-2.27.so   [.] getenv
 	#    4.35%  touch    ld-2.27.so     [.] _dl_fixup
-	perf report --itrace=i1000i --stdio -i ${perfdata} 2>&1 | \
+	perf report --itrace=i20i --stdio -i ${perfdata} 2>&1 | \
 		egrep " +[0-9]+\.[0-9]+% +$1" > /dev/null 2>&1
 }