[proposal/testsuite] require build == host

[Felix Lee]

Daniel Jacobowitz <drow@false.org>:
> Also, what is a host on which GDB works but expect does not?

I remember running build!=host tests in some situations, but I
don't remember enough detail to demonstrate that it was really
necessary.  at the moment I'm mostly using retrograde reasoning:
the build!=host machinery is nontrivial, and it seems unlikely
that people would go to all that trouble if it were easy to just
get build==host working.

the issue isn't memory footprint, it's stuff like OS environment.
one thing that comes to mind: expect won't work if you don't have
ptys, but gdb works fine without ptys.  also, BeOS apparently has
a weird select() call that's hard to work with.

I think most of the build!=host machinery was invented in the
early days of cygwin testing.  it isn't necessary for cygwin now,
but it might be needed elsewhere.
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