test/lib/zombie_hunter.rb: enable zombie hunter for MJIT

I think this issue is solved by Eric Wong.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64427 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/lib/zombie_hunter.rb

@@ -1,8 +1,5 @@
 # frozen_string_literal: true
 
-# There might be compiler processes executed by MJIT
-return if RubyVM::MJIT.enabled?
-
 module ZombieHunter
   def after_teardown
     super