libstdc++: Fix out-of-bound array accesses in testsuite

I fixed some undefined behaviour in string tests in r238609, but I only
fixed the narrow char versions. This applies the same fixes to the
wchar_t ones. These problems were found when testing a patch to make
std::basic_string usable in constexpr.

libstdc++-v3/ChangeLog:

	* testsuite/21_strings/basic_string/modifiers/append/wchar_t/1.cc:
	Fix reads past the end of strings.
	* testsuite/21_strings/basic_string/operations/compare/wchar_t/1.cc:
	Likewise.
	* testsuite/experimental/string_view/operations/compare/wchar_t/1.cc:
	Likewise.