Change invalid test error to avoid windows bot failures

Author: arsenm

llvm/test/tools/llvm-reduce/fail-output-file.test

@@ -1,7 +1,7 @@
 # Python on an empty file will always succeed as interesting
 # RUN: touch %t
 
-# Fail on attempt to write to output directory
-# RUN: not llvm-reduce --delta-passes=instructions -o %s/CannotOpenFile --test %python --test-arg %t %p/Inputs/test-output-format.ll 2>&1 | FileCheck %s
+# Fail on attempt to write output to a directory
+# RUN: not llvm-reduce --delta-passes=instructions -o %p/Inputs --test %python --test-arg %t %p/Inputs/test-output-format.ll 2>&1 | FileCheck  -DMSG=%errc_EISDIR %s
 
-# CHECK: error: opening output file: Not a directory
+# CHECK: error: opening output file: [[MSG]]