tests: add parallel_cp stress test

Modeled after xfstests generic/273
https://github.com/rfjakob/gocryptfs/issues/322

tests/stress_tests/parallel_cp.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+#
+# Simplified xfstests generic/273
+# See https://github.com/rfjakob/gocryptfs/issues/322
+
+echo "deleting old files"
+rm -Rf origin sub_*
+
+SECONDS=0
+echo "creating files with dd"
+mkdir -p origin
+for i in $(seq 1 778) ; do
+	dd if=/dev/zero of=origin/file_$i bs=8192 count=1 status=none
+done
+
+echo -n "cp starting: "
+for i in $(seq 1 100) ; do
+	echo -n "$i "
+	(mkdir sub_$i && cp -r origin sub_$i ; echo -n "$i ") &
+done
+
+echo
+echo -n "cp finished: "
+wait
+echo
+echo "Runtime was $SECONDS seconds"