Bring syntax04 in line with other methods

[mattias-p]

Today all test methods but syntax04 share the same interface. Syntax04 expects its caller to figure out all the nameservers of the zone to be tested and call it repeatedly with each nameserver name while everyone else just takes the name of the zone to be tested. Moreover this exception is not documented anywhere.

Today this works for Zonemaster::Engine::test_zone() and Zonemaster::Engine::test_module() because they call Zonemaster::Engine::Test::Syntax::all() which meets the special expectation of syntax04. But Zonemaster::Engine::test_method() doesn't do anything special for syntax04, so the responsibility is pushed onto the caller. A notable caller is Zonemaster CLI which also doesn't do anything special for syntax04 so the responsibility is pushed onto the user.

The determination of nameserver names for syntax04 should be moved from Zonemaster::Engine::Test::Syntax::all() into Zonemaster::Engine::Test::Syntax::syntax04().

[mattias-p]

Fixed by #1322.