This works around: https://github.com/ziglang/zig/issues/19148
This lets our `test-valgrind` command catch some issues. We'll have to
follow this pattern in more places but I want to do it incrementally so
things keep passing.

I **do not** want to blindly follow this pattern everywhere. I want to
start by focusing in only on the structs that set `undefined` as default
fields that we're also about to test in isolation with Valgrind. Its
just too much noise otherwise and not a general style I'm sure of; it's
worth it for Valgrind though.