perf test: Avoid list test blocking on writing to stdout

Python's json.tool will output the input json to stdout. Redirect to
/dev/null to avoid blocking on stdout writes.

Tested-by: James Clark <james.clark@linaro.org>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Colin Ian King <colin.i.king@gmail.com>
Cc: Howard Chu <howardchu95@gmail.com>
Cc: Weilin Wang <weilin.wang@intel.com>
Cc: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Dapeng Mi <dapeng1.mi@linux.intel.com>
Cc: Athira Jajeev <atrajeev@linux.vnet.ibm.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Veronika Molnarova <vmolnaro@redhat.com>
Link: https://lore.kernel.org/r/20241025192109.132482-5-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

tools/perf/tests/shell/list.sh

@@ -24,8 +24,11 @@ trap trap_cleanup EXIT TERM INT
 
 test_list_json() {
   echo "Json output test"
+  # Generate perf list json output into list_output file.
   perf list -j -o "${list_output}"
-  $PYTHON -m json.tool "${list_output}"
+  # Validate the json using python, redirect the json copy to /dev/null as
+  # otherwise the test may block writing to stdout.
+  $PYTHON -m json.tool "${list_output}" /dev/null
   echo "Json output test [Success]"
 }