Adjust leak test to do as much work in prepare

* The test failed on native otherwise.

Author: eregon

test/mri/tests/ruby/test_rational.rb

@@ -992,7 +992,7 @@ def test_gcd_no_memory_leak
     assert_no_memory_leak([], "#{<<-"begin;"}", "#{<<-"end;"}", limit: 1.2, rss: true)
     x = (1<<121) + 1
     y = (1<<99) + 1
-    1000.times{x.gcd(y)}
+    100.times {1000.times{x.gcd(y)}}
     begin;
       100.times {1000.times{x.gcd(y)}}
     end;