libstdc++: Add dg-output to two tests

These tests are expected to run interactively, with the output checked
by eye. Nobody ever does that, but we can at least use dg-output to
check that the output is as expected.

libstdc++-v3/ChangeLog:

	* testsuite/27_io/objects/char/2.cc: Use dg-output.
	* testsuite/27_io/objects/wchar_t/2.cc: Use dg-output.