Custom test modules

[mattias-p]

Depends on:

• Split test_levels into multiple properties #1336

Zonemaster::Engine::Test/run_for_all() needs to know about all the test modules
it should run and in which order to run them. Today it makes this determination
by parsing a file from disk and prepending the built-in Basic test module. To
run an additional test module you need to include it in the file.

This mechanism should be replaced like so:

• Add a class method to Zonemaster::Engine::Profile that returns the
  Perl package that was provided to the test module registration method.
• Add a class method to Zonemaster::Engine::Profile that returns the of test
  module names. The list should contain an entry for each registered test
  module. Test modules under 'Zonemaster::Engine::Test::' be ordered before any
  others. Within these two groups, test modules should be listed in the order
  they were registered.
• Update Zonemaster::Engine::Test/modules() to call the new Profile
  method instead of using the old parse and prepend algorithm.
• Update the test running methods in Zonemaster::Engine::Test to query the
  Profile module for the ordered set of test module names and their
  corresponding Perl modules.

[mattias-p]

I'm not super-fond of the special handling of modules under 'Zonemaster::Engine::Test::'. The idea is that it should be impossible for 3rd party modules to get loaded before the built-in ones. Perhaps there is a better way to ensure this.