[PATCH] support_create_temp_directory: Align behavior with create_temp_file

[Jonathan Nieder]

Florian Weimer wrote:

> create_temp_file automatically supplies the test directory and the
> XXXXXX suffix.  support_create_temp_directory required the caller to
> specify them, which was confusing.
>
> 2017-11-17  Florian Weimer  <fweimer@redhat.com>
>
> 	* support/temp_file.c (support_create_temp_directory): Use
> 	test_dir and do not rely on the presence of the XXXXXX suffix.
> 	* support/temp_file.h (support_create_temp_directory): Update
> 	comment.
> 	* support/tst-xreadlink.c (do_test): Adjust.

Makes sense.
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>