libstdc++: Fix new <sstream> tests for COW std::string ABI

libstdc++: Fix new <sstream> tests for COW std::string ABI

The std::basic_stringbuf::get_allocator() member is only available for
the SSO std::string ABI.

libstdc++-v3/ChangeLog:

	* testsuite/27_io/basic_istringstream/cons/char/string_view.cc:
	Only check get_allocator() for new string ABI.
	* testsuite/27_io/basic_ostringstream/cons/char/string_view.cc:
	Likewise.
	* testsuite/27_io/basic_stringbuf/cons/char/string_view.cc:
	Likewise.
	* testsuite/27_io/basic_stringstream/cons/char/string_view.cc:
	Likewise.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>