tests: Run perl via 'env'

tests: Run perl via 'env'

The 'perl' binary may not be /usr/bin/perl, depending on the system.
For example, on modern FreeBSD it tends to be /usr/local/bin/perl
instead.

To avoid relying on perl to be in a specific location, just run via
/usr/bin/env instead, so we pick up perl from $PATH instead.

Change-Id: Ic8dc247c82342ff79dfa80426c489ccb8e3e1450
Reviewed-on: https://gerrit.openafs.org/14144
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>