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.
(cherry picked from commit 3749443464)
Before this change the tests ran in about 12m34 and afterwards in
about 6m20, although the real change is more dramatic if you only run
tests from the tests/ subdirectory (as language tests still run serially).
This breaks valgrinding for now, which I intend to fix properly later.
By adding common CLEANFILES and DISTCLEANFILES variables to
common-rules.mk, we can remove these from most other Makefiles, and
also clean files more consistently.
Note that bin_PROGRAMS are already cleaned by 'make clean', so I
removed cases where these were unnecessarily added to CLEANFILES.
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.
