Introduce a way to intentionally expose interface for use in tests

Fixes #1076
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 106f23a..e1208a9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -178,6 +178,7 @@
         ${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