Simplify generation of temporary files

Instead of manuall relying in `$$` being unique, with this patch we
instead use `mktemp` which creates a unique, temporary file which we can
use later. This also removes the possibility that we accidentially
overwrite other existing files in the final location, e.g., from
previous, failed runs.

Via the default behavior of `mktemp` with this patch we also switch from
forced generation of temporary files in `/tmp` to possibly `$TMPDIR` if
set.

btest-diff

@@ -186,7 +186,7 @@ if [ -z "$TEST_DIFF_CANONIFIER" ] || is_binary_mode; then
     TEST_DIFF_CANONIFIER="cat"
 fi
 
-canon_output=/tmp/test-diff.$$.$canon.tmp
+canon_output=$(mktemp -t "test-diff.$canon.tmp.XXXXXX")
 tmpfiles="$tmpfiles $canon_output"
 error=0
 
@@ -201,7 +201,7 @@ if [ $? -ne 0 ]; then
 fi
 
 if [ -n "$baseline" ]; then
-    canon_baseline=/tmp/test-diff.$$.$canon.baseline.tmp
+    canon_baseline=$(mktemp -t "test-diff.$canon.baseline.tmp.XXXXXX")
     tmpfiles="$tmpfiles $canon_baseline"
 
     # Prepare the baseline. When created by a recent btest-diff, we