perf test: test_intel_pt.sh: Stop using backticks

As suggested by shellcheck, stop using backticks.
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20220912083412.7058-6-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/shell/test_intel_pt.sh

@@ -51,7 +51,7 @@ test_system_wide_side_band()
 	perf record -B -N --no-bpf-event -o ${perfdatafile} -e intel_pt//u -C 0 -- taskset --cpu-list 1 uname
 
 	# Should get MMAP events from CPU 1 because they can be needed to decode
-	mmap_cnt=`perf script -i ${perfdatafile} --no-itrace --show-mmap-events -C 1 2>/dev/null | grep MMAP | wc -l`
+	mmap_cnt=$(perf script -i ${perfdatafile} --no-itrace --show-mmap-events -C 1 2>/dev/null | grep MMAP | wc -l)
 
 	if [ ${mmap_cnt} -gt 0 ] ; then
 		return 0