[test] properly 'skip' test on JRuby

an early return still caused ensure to execute,
setting JSON constant to `nil` for later tests!

test/json/json_generator_test.rb

@@ -49,7 +49,6 @@ EOT
   end
 
   def test_remove_const_segv
-    return if RUBY_ENGINE == 'jruby'
     stress = GC.stress
     const = JSON::SAFE_STATE_PROTOTYPE.dup
 
@@ -76,7 +75,7 @@ EOT
     silence do
       JSON.const_set :SAFE_STATE_PROTOTYPE, const
     end
-  end if JSON.const_defined?("Ext")
+  end if JSON.const_defined?("Ext") && RUBY_ENGINE != 'jruby'
 
   def test_generate
     json = generate(@hash)