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

[DJ Delorie]

Joseph Myers <joseph@codesourcery.com> writes:
>> If it can't change, we don't (shouldn't?) need the testroot *at all*,
>> because test-container will never run either.
>
> It's not needed - the reasoning for keeping the rest of the creation (that 
> works fine without needing to be able to run binaries using the new glibc) 
> was more to minimize the differences between the cross and native testing 
> cases (so any problems with the rest of the Makefile rules for creating 
> the testroot would still be detected in cross testing, for example).

My concern is that if we - at any point - create a partial testroot and
later try to use it, we get failures.  I'd really rather defer creating
the testroot until the first time test-container actually *runs* but I
can't find a way to do that with our current Makefile setup.

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.