Extend timeout

On armv7 environment, the timeout is too short
http://rubyci.s3.amazonaws.com/scw-9d6766/ruby-trunk/log/20180319T191706Z.diff.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63151 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_optimization.rb

@@ -707,7 +707,7 @@ class TestRubyOptimization < Test::Unit::TestCase
   end
 
   def test_clear_unreachable_keyword_args
-    assert_separately [], <<-END, timeout: 15
+    assert_separately [], <<-END, timeout: 20
       script =  <<-EOS
         if true
         else