Try renaming sp_log
in the pre-test script, then rename it back in the post-test script.
Pre-test:
EXEC sp_rename 'sp_log', 'sp_log_BACKUP'
CREATE PROCEDURE sp_log AS .... /* your mock sp_log */
Post-test
EXEC sp_rename 'sp_log_BACKUP', 'sp_log'
I'm not sure of a way to reference the stored procedure definitions in variables or something similar. Hopefully renaming the stored procedure as I have suggested is sufficient.