Drop MMTK_HEAP_MAX in test_oom

Should be able to make the test faster.

Author: peterzhu2118

test/mmtk/test_oom.rb

@@ -5,7 +5,7 @@
 module MMTk
   class TestOOM < TestCase
     def test_oom
-      assert_in_out_err([{ "MMTK_HEAP_MAX" => "64MiB" }], <<~RUBY, [], /failed to allocate memory/)
+      assert_in_out_err([{ "MMTK_HEAP_MAX" => "4MiB" }], <<~RUBY, [], /failed to allocate memory/)
         10_000_000.times.map do
           Object.new
         end