Split out parseTestSpec into test-only helpers

tests/SelfTest/helpers/parse_test_spec.hpp

@@ -0,0 +1,15 @@
+
+//              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
+
+#include <catch2/catch_test_spec.hpp>
+
+#include <string>
+
+namespace Catch {
+    TestSpec parseTestSpec( std::string const& arg );
+}