perf test trace: Use shell's -f flag to check if vmlinux exists

To match the style of the existing codebase, no functional changes
were applied.

Signed-off-by: Howard Chu <howardchu95@gmail.com>
Tested-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20250528191148.89118-2-howardchu95@gmail.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

tools/perf/tests/shell/trace_btf_enum.sh

@@ -17,7 +17,7 @@ skip_if_no_perf_trace || exit 2
 
 check_vmlinux() {
   echo "Checking if vmlinux exists"
-  if ! ls /sys/kernel/btf/vmlinux 1>/dev/null 2>&1
+  if [ ! -f /sys/kernel/btf/vmlinux ]
   then
     echo "trace+enum test [Skipped missing vmlinux BTF support]"
     err=2