Simulate OOM for the `try_oom_error` test

We can create the expected error manually, rather than trying to produce
a real one, so the error conversion test can run on all targets. Before,
it was only running on 64-bit and not miri.

In Fedora, we also found that s390x was not getting the expected error,
"successfully" allocating the huge size because it was optimizing the
real `malloc` call away. It's possible to counter that by looking at the
pointer in any way, like a debug print, but it's more robust to just
deal with errors directly, since this test is only about conversion.