Clarify interaction between serializer_test and model_test

This business with changing Fluxor on the fly is pretty confusing.
It would probably be better to have different models, rather than
changing Fluxor's PT config on the fly.

spec/models/fluxor_spec.rb

@@ -1,17 +0,0 @@
-require "rails_helper"
-
-describe Fluxor, type: :model do
-  describe "`be_versioned` matcher" do
-    it { is_expected.to_not be_versioned }
-  end
-
-  describe "Methods" do
-    describe "Class" do
-      describe ".paper_trail.enabled?" do
-        it "returns false" do
-          expect(Fluxor.paper_trail.enabled?).to eq(false)
-        end
-      end
-    end
-  end
-end

test/dummy/app/models/fluxor.rb

@@ -1,3 +1,7 @@
 class Fluxor < ActiveRecord::Base
   belongs_to :widget
+
+  # In test/unit/model_test.rb and test/unit/serializer_test.rb, this is
+  # changed on the fly, which is quite confusing.
+  has_paper_trail
 end

test/unit/serializer_test.rb

@@ -2,12 +2,16 @@ require "test_helper"
 require "custom_json_serializer"
 
 class SerializerTest < ActiveSupport::TestCase
+  setup do
+    # Clean up after test/unit/model_test.rb
+    Fluxor.reset_callbacks :create
+    Fluxor.reset_callbacks :update
+    Fluxor.reset_callbacks :destroy
+    Fluxor.instance_eval "has_paper_trail"
+  end
+
   context "YAML Serializer" do
     setup do
-      Fluxor.instance_eval <<-END
-        has_paper_trail
-      END
-
       @fluxor = Fluxor.create name: "Some text."
 
       # this is exactly what PaperTrail serializes
@@ -34,10 +38,6 @@ class SerializerTest < ActiveSupport::TestCase
         config.serializer = PaperTrail::Serializers::JSON
       end
 
-      Fluxor.instance_eval <<-END
-        has_paper_trail
-      END
-
       @fluxor = Fluxor.create name: "Some text."
 
       # this is exactly what PaperTrail serializes
@@ -77,10 +77,6 @@ class SerializerTest < ActiveSupport::TestCase
         config.serializer = CustomJsonSerializer
       end
 
-      Fluxor.instance_eval <<-END
-        has_paper_trail
-      END
-
       @fluxor = Fluxor.create
 
       # this is exactly what PaperTrail serializes