Run major GC three times to make sure the minor GC reason

Same as 02705b27

http://ci.rvm.jp/results/trunk-random1@phosphorus-docker/2955433
```
  1)
TestGc#test_start_full_mark [/tmp/ruby/v3/src/trunk-random1/test/ruby/test_gc.rb:61]:
Expected :oldmalloc to be nil.
```

test/ruby/test_gc.rb

@@ -56,7 +56,7 @@ def test_start_full_mark
     return unless use_rgengc?
     skip 'stress' if GC.stress
 
-    GC.start # full mark and next time it should be minor mark
+    3.times { GC.start } # full mark and next time it should be minor mark
     GC.start(full_mark: false)
     assert_nil GC.latest_gc_info(:major_by)