[llvm] [test] Avoid writing test output to readonly dir (PR #166404)

[Jordan Rupprecht via llvm-commits]

https://github.com/rupprecht created https://github.com/llvm/llvm-project/pull/166404

Omitting `-o /dev/null` may try to write output to the current dir, which may not have write permissions on some build systems.

>From 113f5b417a2841719935d69a6a1a7fc5bd7fd2a1 Mon Sep 17 00:00:00 2001
From: Jordan Rupprecht <rupprecht at google.com>
Date: Tue, 4 Nov 2025 09:03:55 -0800
Subject: [PATCH] Write to /dev/null

---
 llvm/test/CodeGen/NVPTX/cmpxchg-unsupported-syncscope.err.ll | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/llvm/test/CodeGen/NVPTX/cmpxchg-unsupported-syncscope.err.ll b/llvm/test/CodeGen/NVPTX/cmpxchg-unsupported-syncscope.err.ll
index d3853e2fdaa88..4d81fdc67736d 100644
--- a/llvm/test/CodeGen/NVPTX/cmpxchg-unsupported-syncscope.err.ll
+++ b/llvm/test/CodeGen/NVPTX/cmpxchg-unsupported-syncscope.err.ll
@@ -1,4 +1,4 @@
-; RUN: not llc -mcpu=sm_100a -mtriple=nvptx64 -mattr=+ptx86 %s 2>&1 | FileCheck %s
+; RUN: not llc -mcpu=sm_100a -mtriple=nvptx64 -mattr=+ptx86 %s -o /dev/null 2>&1 | FileCheck %s
 
 ; Test that we get a clear error message when using an unsupported syncscope.