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

JIRA: https://issues.redhat.com/browse/RHEL-78200

upstream
========
commit 6612d4d
Author: Howard Chu <howardchu95@gmail.com>
Date: Wed May 28 12:11:43 2025 -0700

description
===========
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>

Signed-off-by: Anubhav Shelat <ashelat@redhat.com>

Author: AnubhavS1

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