Pretty printer test fixes and improvements

Test that StdUniquePtrPrinter correctly prints std::unique_ptr objects
using the old layout, prior to the PR libstdc++/77990 changes.

The printer test for a valueless std::variant started to fail because
the PR libstdc++/87431 fix meant it no longer became valueless. Change
the test to use a type that is not trivially copyable, so that the
exception causes it to become valueless.

	* testsuite/libstdc++-prettyprinters/compat.cc: Test printer support
	for old std::unique_ptr layout.
	* testsuite/libstdc++-prettyprinters/cxx17.cc: Fix std::variant test
	to become valueless. Add filesystem::path tests.

From-SVN: r267743