[libcxx] [test] Replace _LIBCPP_STD_VER with TEST_STD_VER.

This replaces every occurrence of _LIBCPP_STD_VER in the tests with
TEST_STD_VER. Additionally, for every affected
file, #include "test_macros.h" is being added explicitly if it wasn't
already there.

https://reviews.llvm.org/D26294

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@286007 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/string.view/string.view.cons/default.pass.cpp

@@ -15,9 +15,11 @@
 #include <string_view>
 #include <cassert>
 
+#include "test_macros.h"
+
 template<typename T>
 void test () {
-#if _LIBCPP_STD_VER > 11
+#if TEST_STD_VER > 11
     {
     constexpr T sv1;
     static_assert ( sv1.size() == 0, "" );