I have split up test-skeleton.c into its components.  The new test 
skeleton should be compatible with compilation in C90 mode.  I expect to 
use some of these helper functions for future build support on the host 
(that is, these routines will have to be compiled twice, once against 
the built libc, and once against the host libc).

I converted a few tests to make sure that this works as expected.

I do not propose bulk migration at this point.  Some obscure use cases 
are not supported by the exported hooks.

If you think this goes in the right direction, I will port over my 
container-based nss_files testing, and the resolver test suite to this 
infrastructure.  It also eliminates the need for the hack in my fix for 
bug 16421 I posted earlier.

Tested on aarch64, ppc64le, s390x, x86_64.

Thanks,
Florian