test: consistently use TMPDIR and same name pattern for temp files

tests/test_sha512sum.in

@@ -1,7 +1,7 @@
 #! /bin/bash
 
 # create a randome file
-file=`mktemp`
+file="`mktemp "${TMPDIR:-/tmp}/tmp.XXXXXXXXXX"`" || exit 1
 cat >$file <<EOF
 hello world!
 EOF
@@ -15,12 +15,12 @@ else
 fi
 
 
-outfile1=`mktemp`
+outfile1="`mktemp "${TMPDIR:-/tmp}/tmp.XXXXXXXXXX"`" || exit 1
 @builddir@/grub-shell --files=/boot/grub/file=$file >$outfile1 <<EOF
 sha512sum $grub_file
 EOF
 
-outfile2=`mktemp`
+outfile2="`mktemp "${TMPDIR:-/tmp}/tmp.XXXXXXXXXX"`" || exit 1
 sha512sum $file >$outfile2
 
 SHA1=`cat $outfile1 | tr -d '\n' | cut -f1 -d\ `