libstdc++: Fix some warnings in filesystem tests

libstdc++: Fix some warnings in filesystem tests

There's a -Wunused-but-set-variable warning in operations/all.cc which
can be fixed with [[maybe_unused]].

The statements in operations/copy.cc give -Wunused-value warnings. I
think I meant to use |= rather than !=.

And operations/file_size.cc gets -Wsign-compare warnings.

	* testsuite/27_io/filesystem/operations/all.cc: Mark unused variable.
	* testsuite/27_io/filesystem/operations/copy.cc: Fix typo.
	* testsuite/experimental/filesystem/operations/copy.cc: Likewise.
	* testsuite/27_io/filesystem/operations/file_size.cc: Use correct type
	for return value, and in comparison.
	* testsuite/experimental/filesystem/operations/file_size.cc: Likewise.