[mlir-reduce] Create proper tmp test files (NFC)

[Dinistro]

Summary

This commit ensures that the sh script creates temporary files with
mktmp to ensure they do not collide with existing files. The previous
behaviour caused sporadic permission issues on a multi-user system.

[gysit]

Thanks!

mlir/test/mlir-reduce/failure-test.sh

 #!/bin/sh
+
+# Create temporary files that are automatically deleted after the script's
+# execution.
+stdout_file=$(mktemp /tmp/stdout.XXXXXX)
+stderr_file=$(mktemp /tmp/stderr.XXXXXX)
+
 # Tests for the keyword "failure" in the stderr of the optimization pass
-mlir-opt $1 -test-mlir-reducer > /tmp/stdout.$$ 2>/tmp/stderr.$$
+mlir-opt $1 -test-mlir-reducer > $stdout_file 2> $stderr_file
 
-if [ $? -ne 0 ] && grep 'failure' /tmp/stderr.$$; then
+if [ $? -ne 0 ] && grep 'failure' $stderr_file; then
   exit 1
   #Interesting behavior
 else 
   exit 0
 fi