Add a test that if an offline CPU is requested perf stat will fail.

```
$ perf test -vv "perf stat tests"
101: perf stat tests:
--- start ---
test child forked, pid 46965
Basic stat command test
Basic stat command test [Success]
Null stat command test
Null stat command test [Success]
Offline CPU stat command test (cpu 8)
Offline CPU stat command test [Success]
stat record and report test
stat record and report test [Success]
stat record and script test
stat record and script test [Success]
stat repeat weak groups test
stat repeat weak groups test [Success]
Topdown event group test
Topdown event group test [Success]
Topdown weak groups test
Topdown weak groups test [Skipped event parsing failed]
cputype test
cputype test [Success]
hybrid test
hybrid test [Success]
---- end(0) ----
101: perf stat tests                                                 : Ok
```

Reported-by: Thomas Richter <tmricht@linux.ibm.com>
Closes: https://lore.kernel.org/linux-perf-users/94313b82-888b-4f42-9fb0-4585f9e90080@linux.ibm.com/
Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/tests/shell/stat.sh | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/tools/perf/tests/shell/stat.sh b/tools/perf/tests/shell/stat.sh
index 7885e875caf4..0b2f0f88ca16 100755
--- a/tools/perf/tests/shell/stat.sh
+++ b/tools/perf/tests/shell/stat.sh
@@ -27,6 +27,32 @@ test_null_stat() {
   echo "Null stat command test [Success]"
 }
 
+find_offline_cpu() {
+  for i in $(seq 1 4096)
+  do
+    if [[ ! -f /sys/devices/system/cpu/cpu$i/online || \
+          $(cat /sys/devices/system/cpu/cpu$i/online) == "0" ]]
+    then
+      echo $i
+      return
+    fi
+  done
+  echo "Failed to find offline CPU"
+  exit 1
+}
+
+test_offline_cpu_stat() {
+  cpu=$(find_offline_cpu)
+  echo "Offline CPU stat command test (cpu $cpu)"
+  if ! perf stat "-C$cpu" -e cycles true 2>&1 | grep -E -q "No supported events found."
+  then
+    echo "Offline CPU stat command test [Failed]"
+    err=1
+    return
+  fi
+  echo "Offline CPU stat command test [Success]"
+}
+
 test_stat_record_report() {
   echo "stat record and report test"
   if ! perf stat record -e task-clock -o - true | perf stat report -i - 2>&1 | \
@@ -224,6 +250,7 @@ test_hybrid() {
 
 test_default_stat
 test_null_stat
+test_offline_cpu_stat
 test_stat_record_report
 test_stat_record_script
 test_stat_repeat_weak_groups
-- 
2.52.0.177.g9f829587af-goog