I left something a little ambiguous which I should correct: I have also certainly been testing to see if, in fact, Mormonism belongs in the first category I mentioned: Reasonable, even if not convincingly correct. The failure of anyone to objectively define “testimony of the Holy Ghost” has frustrated any attempts to do so, much to discredit in my eyes those who refer to it.
Let me try again.
The Holy Ghost can communicate in many ways. I know a man who says he heard a voice from heaven declaring the Book of Mormon to be true. Others describe experiences similar to those that occurred on the day of Pentecost (Acts 2:2-4). Some say "our heart burned within us," just as the disciples on the road to Emmaus (Luke 24:32; see also D&C 9:8-9).
Other manifestations are less dramatic, but no less impressive. At times, the Holy Spirit can enlighten one's mind to understand spiritual things (D&C 6:15). At other times, the Spirit can "speak peace to your mind" (D&C 6:23). Or as the Lord told Oliver Cowdery, "behold, I will tell you in your mind and in your heart, by the Holy Ghost, which shall come upon you and which shall dwell in your heart." (D&C 8:2). And sometimes, the Holy Spirit speaks as a "still, small voice" to those who are prepared to listen.
Does that answer your question?
It's a catch phrase for followers of Joseph Smith to identify their reasoning (or lack of) for believing all the "revelations" their prophet fed to the world. It also serves some well as a conversation stopper when the debate isn't going their way.
There are as many testimonies as there are faithful, no two alike. You are trying to catch shadows.