Fix flaky test_latest_gc_info_need_major_by

It's possible for a GC to run between the calls of GC.latest_gc_info,
which would cause the test to fail. We can disable GC so that GC only
triggers manually.

test/ruby/test_gc.rb

@@ -374,6 +374,7 @@ class TestGc < Test::Unit::TestCase
       objects.append(100.times.map { '*' })
     end
 
+    EnvUtil.without_gc do
       # We need to ensure that no GC gets ran before the call to GC.start since
       # it would trigger a major GC. Assertions could allocate objects and
       # trigger a GC so we don't run assertions until we perform the major GC.
@@ -384,6 +385,7 @@ class TestGc < Test::Unit::TestCase
       assert_not_nil(need_major_by)
       assert_not_nil(major_by)
     end
+  end
 
   def test_latest_gc_info_weak_references_count
     assert_separately([], __FILE__, __LINE__, <<~RUBY)