libstdc++: Use dg-additional-files in some I/O tests

libstdc++: Use dg-additional-files in some I/O tests

Use the dg-additional-files directive to declare files that need to be
copied into the test's working directory. This is currently redundant
(as all .tst and .txt files are copied for all tests) but is a step
towards not copying all files.

libstdc++-v3/ChangeLog:

	* testsuite/27_io/basic_filebuf/imbue/char/2.cc: Use
	dg-additional-files.
	* testsuite/27_io/basic_filebuf/imbue/wchar_t/2.cc: Likewise.
	* testsuite/27_io/basic_filebuf/open/char/path.cc: Likewise.
	* testsuite/27_io/basic_filebuf/pbackfail/char/9761.cc:
	Likewise.
	* testsuite/27_io/basic_filebuf/seekoff/char/3-in.cc: Likewise.
	* testsuite/27_io/basic_filebuf/seekoff/char/3-io.cc: Likewise.
	* testsuite/27_io/basic_filebuf/seekpos/char/3-in.cc: Likewise.
	* testsuite/27_io/basic_filebuf/seekpos/char/3-io.cc: Likewise.
	* testsuite/27_io/basic_filebuf/setbuf/char/1.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sgetn/char/3.cc: Likewise.
	* testsuite/27_io/basic_filebuf/underflow/10096.cc: Likewise.
	* testsuite/27_io/basic_fstream/cons/char/path.cc: Likewise.
	* testsuite/27_io/basic_fstream/open/char/path.cc: Likewise.
	* testsuite/27_io/basic_ifstream/assign/1.cc: Likewise.
	* testsuite/27_io/basic_ifstream/cons/move.cc: Likewise.
	* testsuite/27_io/basic_ifstream/cons/char/path.cc: Likewise.
	* testsuite/27_io/basic_ifstream/open/char/path.cc: Likewise.
	* testsuite/27_io/basic_ifstream/open/wchar_t/1.cc: Likewise.
	* testsuite/27_io/objects/char/10.cc: Likewise.
	* testsuite/27_io/objects/char/12048-1.cc: Likewise.
	* testsuite/27_io/objects/char/12048-2.cc: Likewise.
	* testsuite/27_io/objects/char/12048-3.cc: Likewise.
	* testsuite/27_io/objects/char/12048-4.cc: Likewise.
	* testsuite/27_io/objects/char/12048-5.cc: Likewise.
	* testsuite/27_io/objects/wchar_t/12048-1.cc: Likewise.
	* testsuite/27_io/objects/wchar_t/12048-2.cc: Likewise.
	* testsuite/27_io/objects/wchar_t/12048-3.cc: Likewise.
	* testsuite/27_io/objects/wchar_t/12048-4.cc: Likewise.
	* testsuite/27_io/objects/wchar_t/12048-5.cc: Likewise.
	* testsuite/ext/stdio_sync_filebuf/char/12048-1.cc: Likewise.
	* testsuite/ext/stdio_sync_filebuf/char/12048-2.cc: Likewise.
	* testsuite/ext/stdio_sync_filebuf/char/12048-3.cc: Likewise.
	* testsuite/ext/stdio_sync_filebuf/char/12048-4.cc: Likewise.
	* testsuite/ext/stdio_sync_filebuf/wchar_t/12948-1.cc: Likewise.
	* testsuite/ext/stdio_sync_filebuf/wchar_t/12948-2.cc: Likewise.
	* testsuite/ext/stdio_sync_filebuf/wchar_t/12948-3.cc: Likewise.
	* testsuite/ext/stdio_sync_filebuf/wchar_t/12948-4.cc: Likewise.