I'd rather think of this as a bug in the test case, not a bug in 'diff'. 
This is because the test case assumes more about the shell's "<&-" 
construct than what POSIX requires.

The bigger picture here is that invoking programs with stdin closed is 
trouble, and although Gnulib provides several modules to work around the 
trouble these modules are a hassle and many applications don't use them. 
We should encourage platforms like HP-UX that try to work around the 
trouble, instead of having diffutils attempt to simulate the traditional 
but inferior exec semantics. Although the Linux kernel currently 
implements the traditional semantics, in this particular case I think 
I'd rather have diffutils adjust to what the platform provides, rather 
than attempt to simulate Linux behavior even on non-Linux kernels.

So I propose the attached further patch.