This regressed twice over the last two months (new floating point
function versions, and accidental dynamic linking against zlib).
We also want to avoid regressions when merging remote index.
The test is able to do a little bit more than we use in the automated
build (the --sym flag is unused, as is unversioned --lib=GLIBC) but they're
pretty useful when experimenting with how to fix things!
We run the test right at the end, because if it fails we want to be able
to download the binary artifact and inspect it.
Unfortunately by the nature of the test we can only run it when we
produce a build, so currently weekly.
