> Can you replace "_2G" with "_4G" in the decorator for that test?

When I do that, it pushes the memory usage for the test up to 16GB, which is
beyond what the machine can handle. When I tried with 2.5G (_2G * 5 // 4),
that was enough to make it swap heavily (and in the end the test still failed).

As an aside, it turns out the problem with -M being ignored wasn't due to me
being stupid; it seems that -j doesn't pass the memlimit on to subprocesses.
I'll open a separate issue for this.