Avoid a timeout on test_cache_optimization_exponential

The timeout seems too short for some CIs.
http://rubyci.s3.amazonaws.com/debian11-aarch64/ruby-master/log/20221120T012840Z.fail.html.gz

test/ruby/test_regexp.rb

@@ -1676,7 +1676,7 @@ class TestRegexp < Test::Unit::TestCase
 
   def test_cache_optimization_exponential
     assert_separately([], "#{<<-"begin;"}\n#{<<-'end;'}")
-      timeout = #{ EnvUtil.apply_timeout_scale(2).inspect }
+      timeout = #{ EnvUtil.apply_timeout_scale(10).inspect }
     begin;
       Regexp.timeout = timeout