feat: add mock_message function

cmake/Assertion.cmake

@@ -22,3 +22,21 @@ function(assert_false CONDITION)
     message(FATAL_ERROR "expected the condition to be false")
   endif()
 endfunction()
+
+# Mocks the 'message' function.
+#
+# This function mocks the 'message' function by modifying its behavior to store
+# the message into a list variable instead of printing it to the log.
+function(mock_message)
+  function(message MODE MESSAGE)
+    list(APPEND ${MODE}_MESSAGES "${MESSAGE}")
+    set(${MODE}_MESSAGES "${${MODE}_MESSAGES}" PARENT_SCOPE)
+    if("${MODE}" STREQUAL FATAL_ERROR)
+      return()
+    endif()
+  endfunction()
+
+  # Prevents the function from being mocked twice.
+  function(mock_message)
+  endfunction()
+endfunction()