mathext/internal/amos: tolerance on darwin is 1e3× looser on go1.26

In #2085 it was necessary to relax the tolerance of zbknu yr and yi by a factor of 1e3 when bumping the go version to go1.26 on darwin. It would be nice to at least know why.

I could not determine whether this was specifically darwin arm64, but the change was not necessary to get linux on arm64 to pass.