[libc++] Prepare string tests for constexpr

These are the last™ changes to the tests for constexpr preparation.

Reviewed By: Quuxplusone, #libc, Mordante

Spies: Mordante, EricWF, libcxx-commits

Differential Revision: https://reviews.llvm.org/D120951

libcxx/test/std/strings/basic.string/string.modifiers/string_append/pointer.pass.cpp

@@ -18,7 +18,7 @@
 #include "min_allocator.h"
 
 template <class S>
-void
+TEST_CONSTEXPR_CXX20 void
 test(S s, const typename S::value_type* str, S expected)
 {
     s.append(str);