Add test for backing up non-existing files

restic doesn't terminate in this case, this was described by @rubenv
in #90 https://github.com/restic/restic/issues/90#issuecomment-86901128

testsuite/test-backup-non-existing-file.sh

@@ -0,0 +1,10 @@
+set -em
+
+# setup restic
+prepare
+run restic init
+
+# start backup with non existing dir
+run timeout 10s restic.debug backup "${BASE}/fake-data/0/0/"{0,1,foobar,5} && debug "done" || false
+
+cleanup