Stub the PG::Connection class instead of the migration class

Rails 7 changed the way migrations are run.  The migration class
constant is now removed and the file is reloaded between migrations.

This means tests should not use expect/allow on migration stub classes
as the constant stubbed is not the same constant the migration is run against.

Additionally, comparisons of objects created before the migration with ones
after will also not work.  In that case, you can assert by id or other
attributes instead of comparing objects.

Rails also checks association classes are correct and will also not match
expectations.  For example, instead of using host to create or query
the host association, use host_id instead.  This also works with has_manys
such as miq_product_feature_ids.

See https://github.com/rails/rails/pull 42198

spec/migrations/20190617191109_move_awx_credentials_to_authentications_spec.rb

@@ -97,7 +97,8 @@
         :type => "ManageIQ::Providers::EmbeddedAnsible::AutomationManager::VaultCredential"
       )
       expect(auth.manager_ref).to be_nil
-      expect(described_class).to_not receive(:pg_connection)
+      allow(PG::Connection).to receive(:new).and_call_original
+      expect(PG::Connection).to_not receive(:new).with(a_hash_including(:dbname => "awx"))
 
       migrate
     end
@@ -108,7 +109,9 @@
         :type        => "ManageIQ::Providers::AnsibleTower::AutomationManager::GoogleCredential",
         :manager_ref => "1"
       )
-      expect(described_class).to_not receive(:pg_connection)
+
+      allow(PG::Connection).to receive(:new).and_call_original
+      expect(PG::Connection).to_not receive(:new).with(a_hash_including(:dbname => "awx"))
 
       migrate
     end
@@ -121,14 +124,17 @@
         :options     => {:become_method => ""},
         :manager_ref => "1"
       )
-      expect(described_class).to receive(:pg_connection).and_raise(PG::ConnectionBad)
+
+      allow(PG::Connection).to receive(:new).and_call_original
+      expect(PG::Connection).to receive(:new).with(a_hash_including(:dbname => "awx")).and_raise(PG::ConnectionBad)
 
       migrate
     end
 
     context "with an awx database connection" do
       before do
-        allow(described_class).to receive(:pg_connection).with(a_hash_including(:dbname => "awx")).and_return(awx_conn)
+        allow(PG::Connection).to receive(:new).and_call_original
+        allow(PG::Connection).to receive(:new).with(a_hash_including(:dbname => "awx")).and_return(awx_conn)
       end
 
       it "removes unwanted data from options when the credential doesn't exist in the awx database" do