testsuite_hooks.h (__gnu_test::try_mkfifo): Declare it.

	* testsuite/testsuite_hooks.h (__gnu_test::try_mkfifo): Declare it.
	* testsuite/testsuite_hooks.cc (__gnu_test::try_mkfifo): Define
	it.
	* testsuite/27_io/basic_filebuf/close/char/4879.cc: Use try_mkfifo
	and remove Cygwin XFAIL.
	* testsuite/27_io/basic_filebuf/close/char/9964.cc: Likewise.
	* testsuite/27_io/basic_filebuf/imbue/char/13171-2.cc: Likewise.
	* testsuite/27_io/basic_filebuf/open/char/9507.cc: Likewise.
	* testsuite/27_io/basic_filebuf/showmanyc/char/9533-1.cc:
	Likewise.
	* testsuite/27_io/basic_filebuf/underflow/char/10097.cc: Likewise.
	* testsuite/27_io/objects/char/7.cc: Likewise.
	* testsuite/27_io/objects/char/9661-1.cc: Likewise.
	* testsuite/27_io/objects/wchar_t/7.cc: Likewise.
	* testsuite/27_io/objects/wchar_t/9661-1.cc: Likewise.

From-SVN: r75706