[libcxx][test] Include test_workarounds.h where needed

Some tests require `TEST_WORKAROUND_CONSTEXPR_IMPLIES_NOEXCEPT`, but they
did not include the header that defines that macro.

Thanks to Michael Park for the patch.

Differential Revision: https://reviews.llvm.org/D62920

llvm-svn: 362660

libcxx/test/std/utilities/variant/variant.get/get_index.pass.cpp

@@ -27,6 +27,7 @@
 //  variant<Types...>&& v);
 
 #include "test_macros.h"
+#include "test_workarounds.h"
 #include "variant_test_helpers.hpp"
 #include <cassert>
 #include <type_traits>

libcxx/test/std/utilities/variant/variant.get/get_type.pass.cpp

@@ -21,6 +21,7 @@
 // variant<Types...>&& v);
 
 #include "test_macros.h"
+#include "test_workarounds.h"
 #include "variant_test_helpers.hpp"
 #include <cassert>
 #include <type_traits>