[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

llvm-svn: 286007
diff --git a/libcxx/test/std/experimental/string.view/string.view.access/index.pass.cpp b/libcxx/test/std/experimental/string.view/string.view.access/index.pass.cpp
index 2c1bd1d..0cb385e 100644
--- a/libcxx/test/std/experimental/string.view/string.view.access/index.pass.cpp
+++ b/libcxx/test/std/experimental/string.view/string.view.access/index.pass.cpp
@@ -42,7 +42,7 @@
     test ( U"a", 1 );
 #endif
 
-#if _LIBCPP_STD_VER > 11
+#if TEST_STD_VER > 11
     {
     constexpr std::experimental::basic_string_view<char> sv ( "ABC", 2 );
     static_assert ( sv.length() ==  2,  "" );