Try to fix flaky test for Process.warmup

The test sometimes fails with:

```
  1) Failure:
TestProcess#test_warmup_frees_pages [test/ruby/test_process.rb:2749]:
<0> expected but was
<1>.
```

I think there's a page with an object that needs finalization, so run
GC to clear that object.

test/ruby/test_process.rb

@@ -2732,6 +2732,8 @@ EOS
   def test_warmup_frees_pages
     assert_separately([{"RUBY_GC_HEAP_FREE_SLOTS_MAX_RATIO" => "1.0"}, "-W0"], "#{<<~"begin;"}\n#{<<~'end;'}")
     begin;
+      GC.start
+
       TIMES = 10_000
       ary = Array.new(TIMES)
       TIMES.times do |i|