Fix broken sqlite3_mem tests

I put this test in the wrong place and it broke the sqlite3_mem tests.
This test shouldn't run on sqlite3_mem.

activerecord/test/cases/base_prevent_writes_test.rb

@@ -218,11 +218,11 @@ class BasePreventWritesTest < ActiveRecord::TestCase
 
         assert_match %r/\AWrite query attempted while in readonly mode: INSERT /, conn2_error.message
       end
-    end
 
-    test "current_preventing_writes" do
-      ActiveRecord::Base.connection_handler.while_preventing_writes do
-        assert ActiveRecord::Base.current_preventing_writes, "expected connection current_preventing_writes to return true"
+      test "current_preventing_writes" do
+        ActiveRecord::Base.connection_handler.while_preventing_writes do
+          assert ActiveRecord::Base.current_preventing_writes, "expected connection current_preventing_writes to return true"
+        end
       end
     end
   end