Stop test-in-container trying to run other-OS binaries

[DJ Delorie]

> If you want to put this in the "the user needs to do the right thing"
> category wrt manually disabling tests sometimes, I'm OK with that.

To be clear, if the user gets to keep both pieces when he/she breaks it,
the original patch is OK with me.  It's a delicate bit of
dependency-ness and resorting to simple hacks is probably the best we
can do.