To solve bug #70886, the test uses random keys to prevent collisions;
however, this is not guaranteed, and as such it may even collide with
other tests in the shmop test suite. The proper solution would be to
use a single key (which could be randomly generated), but to actually
`shmop_close()` after each `shmop_delete()`. This would, however, not
work on Windows due to bug #65987. Therefore we use three different
keys for now.
We map the POSIX semantics of `IPC_PRIVATE` by creating unnamed file
mapping objects on Windows. While that is not particularly useful for
ext/shmop, which is the only bundled extension which uses `shmget()`,
it may be useful for external extensions.
One of the conditions tested was expected to fail with "Permission denied",
but it doesn't when running as root. The memory segment was also leaked, hence
the split.
