Bug#837137: pkg-perl-autopkgtest: use.t can fall back to source tree

[Niko Tyni]

Package: pkg-perl-autopkgtest
Version: 0.33

Looking into a libnet-easytcp-perl autopkgtest regression, I noticed
that use.t is currently run in the package source directory. When cwd
is on @INC (which it isn't anymore on current sid) the test could fall
back to using that.

In the case of Net::EasyTCP, META.yml has
 name:         EasyTCP

and EasyTCP.pm is in the top source directory, so while cwd was
on @INC, the test succeeded by loading the source version, not the
installed one. This started failing when cwd got removed by default
in perl_5.22.2-4.

We should probably just chdir to / or something like that in use.t to
prevent similar false positives occurring in the future.
-- 
Niko Tyni   ntyni at debian.org