[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.modifiers/remove_prefix.pass.cpp

@@ -39,7 +39,7 @@ void test ( const CharT *s, size_t len ) {
     }
 }
 
-#if _LIBCPP_STD_VER > 11
+#if TEST_STD_VER > 11
 constexpr size_t test_ce ( size_t n, size_t k ) {
     typedef std::basic_string_view<char> SV;
     SV sv1{ "ABCDEFGHIJKL", n };
@@ -67,7 +67,7 @@ int main () {
     test ( U"", 0 );
 #endif
 
-#if _LIBCPP_STD_VER > 11
+#if TEST_STD_VER > 11
     {
     static_assert ( test_ce ( 5, 0 ) == 5, "" );
     static_assert ( test_ce ( 5, 1 ) == 4, "" );