libstdc++: Properly remove temporary directories in filesystem tests

libstdc++: Properly remove temporary directories in filesystem tests

Although these tests use filesystem::remove_all to clean up, that fails
because it uses recursive_directory_iterator which is intentionally
bodged by the custom readdir defined in the test.

Just use POSIX rmdir to clean up. We don't need to use _rmdir or _wrmdir
for Windows, because we'll never reach test02() on targets where the
custom readdir doesn't interpose the one from libc.

libstdc++-v3/ChangeLog:

	* testsuite/27_io/filesystem/iterators/error_reporting.cc: Use
	rmdir to remove directories.
	* testsuite/experimental/filesystem/iterators/error_reporting.cc:
	Likewise.