Add a warning if a diff/patch file is selected and issue status is not testable

When I submit a patch to an issue, then the test bot will ignore it if
- I forget to set it to "needs review"
- the patch does not have a patchy name
- there was more than one file uploaded, and the test bot will only test one of them.

For someone who is using this system for the first time, or has not used it for a few months, or who is just a bit tired, this system is totally intransparent and mysterious. As a result, we see series of comments with patches, all trying to activate the test bot. Here is one issue where I was the lucky one.
This does not just bloat the issue, it also makes the person (me, in this case) look like an idiot. We really should try to avoid that.

So, as a first and simple (I hope) solution, we should change or enhance the generic "Ignored" message, and specify why the patch was ignored, or why it is not considered a patch.
Or at least, display a link next to the comment form, to a page where this stuff is documented.