All REST update path test coverage fixtures contain no-op code and other mistakes

Discovered while working on #2758897, specifically #2758897-23: Move rest module's "link manager" services to serialization module. I looked at the existing upgrade path test coverage and built on top of that. Turns out it's wrong/broken. It only worked by accident.

Three flaws:

1. $connection->insert('key_value')
     ->fields([…])
     ->fields([…])
     ->fields([…])
     ->execute();
   

   Only the first of these is actually executed. I guess it's a huge DX flaw in that DB API too that it doesn't even complain about this… (Which had no consequence in those tests because the module whose upgrade path we were actually testing was always the first one. Hence we can safely delete everything except the first one.)

2. -$extensions['module']['basic_auth'] = 8000;
   -$extensions['module']['rest'] = 8000;
   -$extensions['module']['serialization'] = 8000;
   +$extensions['module']['basic_auth'] = 0;
   +$extensions['module']['rest'] = 0;
   +$extensions['module']['serialization'] = 0;
   

   Those integers do not represent the current schema version, but the module weight. This was misinterpreted. (Which had no consequence in those tests because nothing that is being tested depends on module weight. But it provides a wrong example, so should be fixed.)

3. -  'link_domain' => '~',
   +  'link_domain' => NULL,
   

   A ~ in YAML means NULL in PHP. This was a simple/small oversight. (Which had no consequence in those tests because nothing was testing the link_domain value.)

These mistakes were introduced in:

1. #2308745: Remove rest.settings.yml, use rest_resource config entities
2. #2228141: Add authentication support to REST views
3. #2721595: Simplify REST configuration
4. #2664780: Remove REST's resource- and verb-specific permissions for EntityResource, but provide BC and document why it's necessary for other resources