Test failure: test_insns.rb:423

One test case failed when running on my machine, release build, StickyImmix.

```
Fiber count: 10000 (skipping)
#327 test_insns.rb:423:  $~ != true   #=> "" (expected "true")  putobject
FAIL 1/1425 tests failed
```

But I am not able to reproduce it.  I am taking a note here in case the error appears again in the future.