Use the new licence header in SelfTest's cpp files

tests/SelfTest/UsageTests/ToStringVariant.tests.cpp

@@ -1,3 +1,11 @@
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE_1_0.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
+
 #define CATCH_CONFIG_ENABLE_VARIANT_STRINGMAKER
 #include <catch2/catch_test_macros.hpp>