[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/experimental/optional/optional.nullops/less_equal.pass.cpp

@@ -15,9 +15,11 @@
 
 #include <experimental/optional>
 
+#include "test_macros.h"
+
 int main()
 {
-#if _LIBCPP_STD_VER > 11
+#if TEST_STD_VER > 11
     using std::experimental::optional;
     using std::experimental::nullopt_t;
     using std::experimental::nullopt;