Bug 22238 – Intermittent failure in druntime forkgc2 test

druntime's `forkgc2` test sometimes fails, with the following error: --- forkgc2(91895,0x700000104000) malloc: *** error for object 0x7fe3c3403130: double free --- This was recently observed to occur in an auto-tester job running on 64-bit Mac OS X for Phobos PR 8208 [1]. Re-running the tests caused the error to vanish. It is possible that this error has a similar cause to the one described in issue 22222, since the way `fork` and `exit` are used in `forkgc2` is very similar to the problematic usage pattern described in that issue. [1] https://github.com/dlang/phobos/pull/8208

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