Try to fix flaky test_warmup_frees_pages

This test sometimes fails with:

```
  1) Failure:
TestProcess#test_warmup_frees_pages [test/ruby/test_process.rb:2750]:
<202> expected but was
<203>.
```

test/ruby/test_process.rb

@@ -2742,6 +2742,9 @@ EOS
       ary.clear
       ary = nil
 
+      # Disable GC so we can make sure GC only runs in Process.warmup
+      GC.disable
+
       total_pages_before = GC.stat(:heap_eden_pages) + GC.stat(:heap_allocatable_pages)
 
       Process.warmup