tests/Makefile.am assumes the test image is located in
tests/syslinux/, but we actually created it in tests/. Fix the
location, and also ensure the test will clean it up after running
successfully.
Instead of hardcoding the location of perl (assuming it is installed in
/usr), use /usr/bin/env to run it, and thus picking it from $PATH.
This makes it possible to run these scripts also on installations with
perl in a different prefix than /usr.
Also, given that we want enable warnings on scripts, turn the -w
previously in shebang to explicit "use warnings;" in scripts which
didn't have it before.
