libstdc++: Move test that depends on wchar_t I/O to wchar_t sub-directory

This fixes a FAIL when --disable-wchar_t is used.

libstdc++-v3/ChangeLog:

	* testsuite/27_io/basic_filebuf/close/81256.cc: Moved to...
	* testsuite/27_io/basic_filebuf/close/wchar_t/81256.cc: ...here.

diff --git a/libstdc++-v3/testsuite/27_io/basic_filebuf/close/81256.cc b/libstdc++-v3/testsuite/27_io/basic_filebuf/close/wchar_t/81256.cc
similarity index 100%
rename from libstdc++-v3/testsuite/27_io/basic_filebuf/close/81256.cc
rename to libstdc++-v3/testsuite/27_io/basic_filebuf/close/wchar_t/81256.cc