Introduce a way to intentionally expose interface for use in tests

Fixes #1076

CMakeLists.txt

@@ -178,6 +178,7 @@ set(INTERNAL_HEADERS
         ${HEADER_DIR}/internal/catch_timer.h
         ${HEADER_DIR}/internal/catch_tostring.h
         ${HEADER_DIR}/internal/catch_totals.h
+        ${HEADER_DIR}/internal/catch_user_interfaces.h
         ${HEADER_DIR}/internal/catch_version.h
         ${HEADER_DIR}/internal/catch_wildcard_pattern.h
         ${HEADER_DIR}/internal/catch_windows_h_proxy.h

include/catch.hpp

@@ -43,6 +43,7 @@
 #  endif
 #endif
 
+#include "internal/catch_user_interfaces.h"
 #include "internal/catch_tag_alias_autoregistrar.h"
 #include "internal/catch_test_registry.h"
 #include "internal/catch_capture.hpp"

include/internal/catch_user_interfaces.h

@@ -0,0 +1,18 @@
+/*
+ *  Created by Martin on 21/11/2017.
+ *
+ *  This file collects declaration that we want to expose to test files.
+ *  These declarations are expected to be duplicated elsewhere,
+ *  together with their implementation.
+ *
+ *  Distributed under the Boost Software License, Version 1.0. (See accompanying
+ *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ */
+#ifndef TWOBLUECUBES_CATCH_USER_INTERFACES_H_INCLUDED
+#define TWOBLUECUBES_CATCH_USER_INTERFACES_H_INCLUDED
+
+namespace Catch {
+    unsigned int rngSeed();
+}
+
+#endif // TWOBLUECUBES_CATCH_USER_INTERFACES_H_INCLUDED