Extract AR::Persistence#becomes's test code out from base_test.rb

The method got extracted out from AR::Base in commit
d916c62cfc, but the tests never did.

activerecord/test/cases/base_test.rb

@@ -1228,19 +1228,6 @@ class BasicsTest < ActiveRecord::TestCase
     assert_no_queries { assert true }
   end
 
-  def test_becomes
-    assert_kind_of Reply, topics(:first).becomes(Reply)
-    assert_equal "The First Topic", topics(:first).becomes(Reply).title
-  end
-
-  def test_becomes_includes_errors
-    company = Company.new(:name => nil)
-    assert !company.valid?
-    original_errors = company.errors
-    client = company.becomes(Client)
-    assert_equal original_errors, client.errors
-  end
-
   def test_silence_sets_log_level_to_error_in_block
     original_logger = ActiveRecord::Base.logger
 

activerecord/test/cases/persistence_test.rb

@@ -139,6 +139,19 @@ class PersistenceTest < ActiveRecord::TestCase
     end
   end
 
+  def test_becomes
+    assert_kind_of Reply, topics(:first).becomes(Reply)
+    assert_equal "The First Topic", topics(:first).becomes(Reply).title
+  end
+
+  def test_becomes_includes_errors
+    company = Company.new(:name => nil)
+    assert !company.valid?
+    original_errors = company.errors
+    client = company.becomes(Client)
+    assert_equal original_errors, client.errors
+  end
+
   def test_delete_many
     original_count = Topic.count
     Topic.delete(deleting = [1, 2])