libstdc++: Fix comments in std::forward_list tests

libstdc++: Fix comments in std::forward_list tests

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/forward_list/operations/1.cc: Fill in
	placeholders in comments.
	* testsuite/23_containers/forward_list/operations/2.cc:
	Likewise.
	* testsuite/23_containers/forward_list/operations/3.cc:
	Likewise.
	* testsuite/23_containers/forward_list/operations/4.cc:
	Likewise.
	* testsuite/23_containers/forward_list/operations/5.cc:
	Likewise.
	* testsuite/23_containers/forward_list/operations/6.cc:
	Likewise.
	* testsuite/23_containers/forward_list/operations/7.cc:
	Likewise.