libstdc++: Adjust expected errors in tests when compiled as C++20

libstdc++-v3/ChangeLog:

	* testsuite/20_util/scoped_allocator/69293_neg.cc: Add dg-error
	for additional errors in C++20.
	* testsuite/20_util/specialized_algorithms/memory_management_tools/destroy_neg.cc:
	Likewise.
	* testsuite/20_util/uses_allocator/69293_neg.cc: Likewise.
	* testsuite/27_io/filesystem/path/io/dr2989.cc: Likewise.