Fix testing with read-only source directory

[Jonathan Nieder]

Joseph Myers wrote:

> Three tests fail with a read-only source directory because they try to
> write into the source directory.  None of these write into it in a way
> that should actually be problematic for concurrent builds sharing the
> same writable source directory, but avoiding any writing into the
> source directory (from testing, or from building glibc if the source
> timestamps are properly ordered) is still a good idea, as being able
> to build with read-only sources helps make sure there isn't anything
> that could cause problems for concurrent builds.
>
> This patch changes the tests in question to use either /tmp or the
> build directory to write their temporary files (or to test O_TMPFILE,
> as applicable).
>
> Tested for x86_64.
>
> 2017-12-15  Joseph Myers  <joseph@codesourcery.com>
>
> 	* io/Makefile (tst-open-tmpfile-ARGS): New variable.
> 	* posix/tst-mmap-offset.c (fname): Use /tmp.
> 	* stdlib/tst-setcontext3.sh (tempfile): Use ${objpfx}.

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Thanks for fixing it.

[...]
> --- a/posix/tst-mmap-offset.c
> +++ b/posix/tst-mmap-offset.c
> @@ -29,7 +29,7 @@
>  
>  static int fd;
>  static long int page_shift;
> -static char fname[] = "tst-mmap-offset-XXXXXX";
> +static char fname[] = "/tmp/tst-mmap-offset-XXXXXX";

It would be nice if this kind of test respected $TMPDIR, but that's an
orthogonal problem --- what this patch does is consistent with the
other tests in this dir.