Warn if gen-test.sh would write new files into non-empty dir.

Signed-off-by: Stefan Westerfeld <stefan@space.twc.de>

src/gen-test.sh

 #!/bin/bash
 
 mkdir -p test
+if [ ! -z "$(ls test)"  ]; then
+  echo test dir not empty
+  exit 1
+fi
 
 seq=1
 cat test_list | while read f