nixos/testing: add deprecation notice for Perl VM tests

Most VM tests have been migrated to use the python test driver
(introduced in #71684), the migration is tracked in #72828 (which also
thankfully uncovered and fixed many currently broken tests)

While increasing the acceptance and adoption of NixOS integration tests
by using a more popular language, there was also nobody willing to do
larger refactors in the currently very convoluted test infrastructure.

We plan to remove the perl infrastructure between the 20.03 and 20.09
release, to be able to do these refactorings.

Some people might be using Perl tests in their internal CI, so print a
warning for 20.03, and give users time to move to the python testing
infrastructure.