Test module registration

[mattias-p]

Depends on:

• Define default message severities inside test modules #1334

When Zonemaster::Engine::Profile is loaded, the test_cases and test_levels
properties of the default profile should start out empty.

The Zonemaster::Engine::Profile module should provide a class method for test
module registration. When a test module is registered the Profile module should:

• make sure that no test module with the same test module name is already
  registered,
• collect the set of test case names from the test module and add them to the
  test_cases property of the default profile,
• collect the set of message tags and default severities from the test module
  and add them to the test_levels property of the default profile.

The Zonemaster::Engine::Test module should register all built-in test modules
with Zonemaster::Engine::Profile module. The Basic test module should be
registered first and the remaining ones should be registered in case-insensitive
lexicographic order.