libstdc++: Only use dynamic_cast in tests when RTTI is enabled

libstdc++: Only use dynamic_cast in tests when RTTI is enabled

libstdc++-v3/ChangeLog:

	* testsuite/27_io/basic_istringstream/rdbuf/char/2832.cc: Use
	static_cast when RTTI is disabled.
	* testsuite/27_io/basic_istringstream/rdbuf/wchar_t/2832.cc:
	Likewise.
	* testsuite/27_io/basic_ostringstream/rdbuf/char/2832.cc:
	Likewise.
	* testsuite/27_io/basic_ostringstream/rdbuf/wchar_t/2832.cc:
	Likewise.
	* testsuite/27_io/basic_stringstream/str/char/2.cc:
	Likewise.
	* testsuite/27_io/basic_stringstream/str/wchar_t/2.cc:
	Likewise.