[libcxx] [test] Don't require path::operator(string_type&&) to be noexcept

[mstorsjo]

Summary

Mark this as a libcpp specific test; the standard doesn't say that this method should be noexcept.

libcxx/test/std/input.output/filesystems/class.path/path.member/path.assign/source.pass.cpp

void RunStringMoveTest(const char* Expect) {
   {
     DisableAllocationGuard g; ((void)g);
     path& pr = (p = std::move(ss));
     assert(&pr == &p);
   }
   assert(p == Expect);
   {
     // Signature test
-    ASSERT_NOEXCEPT(p = std::move(ss));
+    LIBCPP_ONLY(ASSERT_NOEXCEPT(p = std::move(ss)));
   }
 }
 
 int main(int, char**) {
   for (auto const& MS : PathList) {
     RunTestCase<char>(MS);
     RunTestCase<wchar_t>(MS);
     RunTestCase<char16_t>(MS);
     RunTestCase<char32_t>(MS);
     RunStringMoveTest(MS);
   }
   test_sfinae();
 
   return 0;
 }