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.
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.
It's reasonable to expect megabytes will be freed up and given back to
the host. Previously we just tested that at least 1 block was
released. This test checks that at least 1000 blocks are released
(since stat(2) returns blocks of 512 bytes: 512 bytes * 1000 = approx 0.5 MB).
