Fix flaky test for Process.warmup

The test sometimes fails with:

```
  1) Failure:
TestProcess#test_warmup_run_major_gc_and_compact [test/ruby/test_process.rb:2712]:
<2> expected but was
<3>.
```

test/ruby/test_process.rb

@@ -2706,6 +2706,9 @@ EOS
   def test_warmup_run_major_gc_and_compact
     assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
     begin;
+      # Run a GC to ensure that we are not in the middle of a GC run
+      GC.start
+
       major_gc_count = GC.stat(:major_gc_count)
       compact_count = GC.stat(:compact_count)
       Process.warmup