64-bit GitHub Actions builds need t0021 to be run separately

[me-and]

The test seems to pass when run stand-alone, but hangs when run as part of prove in GitHub Actions.

I'm guessing the issue might be related to #47: if the test fails on 32-bit builds before it gets as far as whatever step is hanging, then that would explain the difference between 32- and 64-bit behaviour. If that's the case, it would probably indicate that the patch added for #52 to disable t0021.12 for all builds might mean this ticket is essentially a duplicate of those two…

[me-and]

...this seems to be hanging regardless of how I run it now. No idea what changed :(