[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

libcxx/test/std/strings/string.view/string.view.iterators/begin.pass.cpp

@@ -58,7 +58,7 @@ int main()
     test(u32string_view{U"123"});
 #endif
 
-#if _LIBCPP_STD_VER > 11
+#if TEST_STD_VER > 11
     {
     constexpr string_view       sv { "123", 3 };
     constexpr u16string_view u16sv {u"123", 3 };