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

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.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D145054

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

@@ -1,8 +1,14 @@
 #!/bin/sh
-# Tests for the keyword "failure" in the stderr of the optimization pass
-mlir-opt $1 -test-mlir-reducer > /tmp/stdout.$$ 2>/tmp/stderr.$$
 
-if [ $? -ne 0 ] && grep 'failure' /tmp/stderr.$$; then
+# 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 > $stdout_file 2> $stderr_file
+
+if [ $? -ne 0 ] && grep 'failure' $stderr_file; then
   exit 1
   #Interesting behavior
 else