Define default message severities inside test modules

[mattias-p]

Depends on:

• #1333

Add a new class method (e.g. all_messages()) to the test module interface for returning a nested hash of test case name => message tag => default severity entry for each message tag that can be emitted by the test module.

Add a new class method (e.g. all_test_cases()) to the test module interface for returning a list of all test cases implemented by the test module.

Deprecate the metadata() methods and recommend using all_test_cases and/or all_messages instead.

Replace internal calls to metadata() where possible. N.b. this call is still needed in generateTestMessages.pl util in the zonemaster/zonemaster repo.