Make sure isolation tests are properlly parallelized

When `parallelize` is used, and the ActiveSupport::Testing::ParallelizeExecutor
is being set, we can't call `parallelize_me!` ourselves since the
parallel executor might not be set to run tests in parallel.

In fact, maybe we should stop calling `parallelize_me!` altogether here
and rely on `parallelize` to do the right thing, but this might break
test classes that are using `ActiveSupport::Testing::Isolation` outside
without calling `parallelize`.

Author: rafaelfranca

activesupport/lib/active_support/testing/isolation.rb

@@ -9,7 +9,7 @@ module Isolation
 
       def self.included(klass) # :nodoc:
         klass.class_eval do
-          parallelize_me!
+          parallelize_me! unless Minitest.parallel_executor.is_a?(ActiveSupport::Testing::ParallelizeExecutor)
         end
       end