perf tests perf_dat _converter_json: Use quoting to avoid word splitting

Running shellcheck on test_perf_data_converter_json.sh throws
below warning:

   In tests/shell/test_perf_data_converter_json.sh line 42:
   	if [ $(cat "${result}" | wc -l) -gt "0" ] ; then
                ^------------------------^ SC2046 (warning): Quote this to prevent word splitting.

Fixed the warning by adding quotes to avoid word splitting.

ShellCheck result with patch:
    # shellcheck -S warning test_perf_data_converter_json.sh
    #

Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Disha Goel <disgoel@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: linuxppc-dev@lists.ozlabs.org
Link: https://lore.kernel.org/r/20230709182800.53002-9-atrajeev@linux.vnet.ibm.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>