libstdc++: Update documentation about copyright and GPL notices in tests

libstdc++: Update documentation about copyright and GPL notices in tests

There is no need to require FSF copyright for tests that are just
"self-evident" ways to check the API and behaviour of the library.
This is consistent with tests for the compiler, which do not have
copyright and licence notices either.

libstdc++-v3/ChangeLog:

	* doc/xml/manual/test.xml: Remove requirement for copyright and
	GPL notice in tests.
	* doc/html/manual/test.html: Regenerate.