update-copyright self-tests bug

[Simon Josefsson]

This problem came up for Libidn but it seems to be a problem with the
update-copyright self check, it fails under Cygwin.

> * Cygwin 1.7.9
>
> Fails already in the gnulib tests:
>
> 1 of 36 tests failed
>
> --- - 2011-11-23 01:13:33.194375000 +0100
> +++ update-copyright.test-ex-stderr   2011-11-23 01:13:33.099750000 +0100
> @@ -1,2 +1,4 @@
>  update-copyright.test-ex.4: warning: copyright statement not found
> +update-copyright.test-ex.4.bak: warning: copyright statement not found
>  update-copyright.test-ex.5: warning: copyright statement not found
> +update-copyright.test-ex.5.bak: warning: copyright statement not found
> FAIL: test-update-copyright.sh

Any ideas where the *.bak files come from?  I can't find anything in
test-update-copyright.sh that creates them, so I suspect that
Cygwin/Windows somehow creates backup files for some reason?  Is there
any way to disable that?  Any other ideas?

Meanwhile, I'm disabling this self-test in Libidn since it doesn't
affect the stability of the rest of the package.  Generally, this
self-tests seems to be in a somewhat different class than usual
self-tests, more like a maintainer-check thing?

/Simon